To solve the equation \(x^2 = 81\) by inspection, we can take the square root of both sides. The equation implies that:
\[ x = \pm \sqrt{81} \]
Calculating the square root, we find:
\[ \sqrt{81} = 9 \]
Thus, the solutions are:
\[ x = 9 \quad \text{and} \quad x = -9 \]
Therefore, the complete solution set is:
\[ {-9, 9} \]
The correct response is:
\({-9, 9}\)