The trick here is to find the height.
The difference between the two base lengths is 15 - 9 = 6
There is a 3 inch difference between the end of the shortest base and the end of the longest base at each end of the shortest base. You are going to have to read that sentence a couple of times.
Because the base angles are 45°, the height = the 3 inch difference between the ends of the 2 bases.
L1 = 9
L2 = 15
h = 3
Area = (L1 + L2)*h/2
Area = (9 + 15) * 3 /2
Area = (24) * 3/2
Area = 72/2
Area = 36 <<<< answer