The correct identification of Hank's error is: He did not set up the Pythagorean Theorem correctly. The hypotenuse should be isolated on one side of the equation.
In a right triangle, according to the Pythagorean Theorem, the relationship between the lengths of the sides is given by the equation \( a^2 + b^2 = c^2 \), where \( c \) is the length of the hypotenuse. In Hank's case, the correct setup should have been:
\[ 9^2 + (40 , \text{cm})^2 = c^2 \]
Which translates to:
\[ 81 + 1600 = c^2 \]
Thus:
\[ c^2 = 1681 \]
And then \( c = \sqrt{1681} \), which gives the correct hypotenuse length.