The question is tricky. But any way the longest side of a right triangle is always the hypotenuse. So we know the hypotenuse if 15 cm.
Now, consider the Pythagorean theorem:
a^2 + b^2 = h^2, where a and b are the sides of a right triangle and h is the hypotenuse.
We know one side is 12cm and the hypotenuse is 15 cm, so:
12^2 + b^2 = 15^2, or 144 + b^2 = 225
subtract 144 from either side to get
b^2 = 225 - 144 = 81
now take the square root of either side to get
b = 9
The third side is 9 cm
