Help with question 9 please

Volume of a prism = Bh

B = area of the base
h = height

B = 1/2 (base1 + base2) x height
B = 1/2 (9+6) x 8
B = 1/2 (15) x 8
B = 4 x 15
B = 60

Volume = 720
720 = B x h
720 = 60 x h
h = 12

So the height or length is 12mm.

Surface Area = 2B + 2P

Where 
B is the area of the base
P is the perimeter of the base

We already know that B= 60

P = 9 + 10 + 6 + 8
P = 33

So

SA = 2*60 + 2*33
SA = 120 + 66
SA = 186

The surface area is equal to 186mm^2.

B? ? i'm most likely wrong but i think it would be b