we can subtract the trapezium (white part) formed after from the trapezium (including white and grey) to get area of striped part
area of trapezium is [tex]\frac H2(a+b)[/tex] where $a$ and $b$ are parallel sides.
for bigger trapezium, $h=4$ parallel sides are $5$ and $5$
hence area is $\frac{4(5+5)}{2}=20$
similarily area of white trapezium, $\frac{4(3+3)}{2}=12$
and area of striped part is $20-12=8$