we know that
1 ft is equal to 12 in
step 1
dimensions of plywood required
(12+3+3) x (18+3+3)------> 18 in x 24 in
step 2
convert inches to feet
18 in-------> 18/12=1.5 ft
24 in------> 24/12=2 ft
step 3
area of plywood required
A=b*h------> 1.5*2------> A=3 ft²
the answer is
3 ft²
