if by decomposition you mean breaking it ino parts then
break it itno 2 triangles and 1 rectangle
left to right
triangle with base y and height h
h=10
y=3
area of traignel=1/2bh
aera=1/2(10)(3)
area=5(3)
area=15
rectangel of legnth x and height h
area=legnth timwe width
x=8
h=10
area=8 times 10
aera=80
last triangle
should be same as other sinces same base (y) and same height (h)
15
add
15+80+15=120
