Shawna's error occurred in Step 2.
In Step 1, she correctly set up the equation \( a^2 + 72 = 252 \).
In Step 2, she should have correctly simplified the equation from Step 1. The correct calculation is: \[ a^2 + 72 = 252 \] Subtracting 72 from both sides gives: \[ a^2 = 252 - 72 \] \[ a^2 = 180 \] Instead, she wrote \( a^2 + 14 = 50 \), which is incorrect.
Therefore, the correct answer is: Step 2. She incorrectly squared the numbers.