Ttdfjt

begintikzpicture
draw[gray!50, very thick,, fill = blue!30,rounded corners]
(0,0) rectangle (2.5,1.5)node[pos=.5]$beginarrayctexttextcolorblue!75textbfCombinatorial \ texttextcolorblue!75textbfProblemendarray$
;
draw[->, thick, blue] (2.7,0.75) -- (3.8,0.75)
;
draw[gray!50, very thick, fill = blue!30,rounded corners]
(4,0) rectangle (6.5,1.5)node[pos=.5]$beginarrayctexttextcolorblue!75textbfBoolean \ texttextcolorblue!75textbfExpression(s)endarray$
;
draw[->, thick, blue] (6.7,0.75) -- (7.8,0.75)
;
draw[gray!50, very thick, fill = blue!30,rounded corners]
(8,0) rectangle (10.5,1.5)node[pos=.5]$beginarrayctexttextcolorblue!75textbfConjuctive \ texttextcolorblue!75textbfNormal Formsendarray$
;
draw[->, thick, blue] (9.75,-0.2) -- (9.75,-0.-1.8)
;
draw[gray!50, very thick,, fill = blue!30,rounded corners]
(0,-3.5) rectangle (2.5,-2)node[pos=.5]$beginarrayctexttextcolorblue!75textbfProblem \ texttextcolorblue!75textbfSolutionendarray$
;
draw[<-, thick, blue] (2.7,-2.75) -- (3.8,-2.75)
;
draw[gray!50, very thick, fill = blue!30,rounded corners]
(4,-3.5) rectangle (6.5,-2)node[pos=.5]$beginarrayctexttextcolorblue!75textbfAssignment of \ texttextcolorblue!75textbfVariable(s)endarray$
;
draw[<-, thick, blue] (6.7,-2.75) -- (7.8,-2.75)
;
draw[gray!50, very thick, fill = blue!30,rounded corners]
(8,-3.5) rectangle (10.5,-2)node[pos=.5]$beginarrayctexttextcolorblue!75textbfSAT \ texttextcolorblue!75textbfSolversendarray$
;
endtikzpicture

You could use the single arrow shape from the shapes.arrows library. Also things may become easier if you use e.g. a matrix.

documentclass[tikz,border=3.14mm]standalone
usepackageamsmath
usetikzlibrarymatrix,shapes.arrows,shadows.blur
begindocument
begintikzpicture[box/.style=align=center,text width=2.7cm,
text=blue!75,fill = blue!30,rounded corners,draw=gray!50, 
very thick,font=bfseries,minimum height=1.5cm,minimum width=2.7cm,blur shadow,
arr/.style=single arrow,fill=blue!40,draw=blue,thick,blur shadow,minimum height=2em,
single arrow head extend=3pt]
matrix[matrix of nodes,nodes=box,anchor=center,column sep=1.2cm,row sep=1.2cm] (mat) 
 Combinatorial \ 
 Problem
 &
 Boolean \ 
 Expression(s)
 &
 Conjuctive \ 
 Normal Forms
 \
 Problem \ 
 Solution
 &
 Assignment of \ 
 Variable(s)
 &
 SAT \ 
 Solvers \
;

path foreach X [count=Y] in 2,3 
 (mat-1-Y) -- (mat-1-X) node [midway,sloped,arr] 
 (mat-2-Y) -- (mat-2-X) node [midway,sloped,arr,shape border rotate=180] ;
path (mat-1-3) -- (mat-2-3) node [midway,sloped,arr] ;
endtikzpicture
enddocument

Or

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarymatrix,shapes.arrows,shadows.blur
definecolormyblueRGB203, 216, 240
definecolorarrbRGB97, 128, 183
definecolorarrcRGB71, 95, 139
definecolornodetRGB97, 127, 180
begindocument
begintikzpicture[box/.style=align=center,text width=2.7cm,
text=nodet,fill=myblue,rounded corners,draw=gray!50, 
very thick,font=bfseriessffamily,minimum height=2cm,minimum width=2.7cm,blur shadow,
arr/.style=single arrow,fill=arrb,draw=arrc,thick,blur shadow,minimum height=2em,
single arrow head extend=3pt]
matrix[matrix of nodes,nodes=box,anchor=center,column sep=1.2cm,row sep=1.2cm] (mat) 
 Combinatorial \ Problem
 &
 Boolean \ Expression(s)
 &
 Conjuctive \ Normal Forms
 \
 Problem \ Solution
 &
 Assignment of \ Variable(s)
 &
 SAT \ Solvers 
 \
;

path foreach X [count=Y] in 2,3 
 (mat-1-Y) -- (mat-1-X) node [midway,sloped,arr] 
 (mat-2-Y) -- (mat-2-X) node [midway,sloped,arr,shape border rotate=180] ;
path (mat-1-3) -- (mat-2-3) node [midway,sloped,arr] ;
endtikzpicture
enddocument