To solve for \( z \) in the equation \( z^2 = 264.950495290000 \), we take the square root of both sides:
\[ z = \sqrt{264.950495290000} \]
Calculating the square root:
\[ z \approx 16.3 \]
Thus, rounded to the nearest tenth, the answer is:
\[ z \approx 16.3 \]