Answer:
12
Explanation:
A = (a+b)/2 x h
Where : a is the longer base, b is the shorter base, and h is the height
Replace the numbers, we get : (15+b)/2 x 6 = 81
(15+b)/2 = 81/6 = 27/2
=> 15 + b = 27
b = 27 - 15 = 12
So, the shorter base's length is 12.