The longest side of a right triangle is 15 cm long another side is 12 cm long what is the length of the third side

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

The scale factor of what ?