Why the Formulas Work · Medium problem

A right triangle has legs $9$ and $12$. Fay finds the hypotenuse:

Step 1. $c = \sqrt{9^2 + 12^2}$ Step 2. $= \sqrt{81} + \sqrt{144}$ Step 3. $= 9 + 12 = 21$

Which step contains the first error? Answer with the step number.