To solve the equation \(x^2 = 81\) by inspection, we can take the square root of both sides. The square root of 81 is 9, so we have:
\[ x = 9 \quad \text{or} \quad x = -9 \]
Therefore, the complete solution set is:
\({ -9, 9 }\)
So the correct response is:
{−9, 9}