Hank's error is that he did not set up the Pythagorean Theorem correctly. The hypotenuse should be isolated on one side of the equation.
In the Pythagorean theorem, for a right triangle with legs \( a \) and \( b \), and hypotenuse \( c \), the relationship is given by:
\[ c^2 = a^2 + b^2 \]
If Hank is calculating the hypotenuse but has equations that do not align with this structure, it indicates a misrepresentation of the theorem.