Use Pythagoras theorem:
C^2 = a squared + b squared
(We label the diagonal, the hypotenuse, as c and either of the two others sides as a and b)
Fill in values
15^2 = 12^2 + b^2
225 = 144 +b^2
225 - 144 = b^2
81 = b^2
To find b we find the square root of 81
So b = 9
