What is the length of the diagonal if a 10 cm by 15cm rectangle?

In other words, if a rectangle is 10 cm by 15 cm, what is the length of the diagonal? Use the Pythagorean Theorem. a^2+b^2=c^2, where c is the measure of the hypotenuse.

20 cm i think

I am not sure but my thinking is

The rectangle becomes 2 30 60 90 triangles and 10 would be the short side making it 20 because the short side is a and the hypotenuse is 2s.