The mean is found by dividing the sum of all terms by the number of terms present in the set.
Set up the equation:

The mean of this sequence is
5.
A^2 + b^2 = c^2
9^2 + b^2 = 15^2
81 + b^2 = 225
225 - 81 = b^2
144 = b^2
square root of 144 = 12
b = 12
This is how i worked it out.
Good Luck