Answer
$x\lt-4$ or $x\gt 4$
Work Step by Step
We know that the solution to the inequality:
$x^2\gt a$
Is equivalent to:
$x\lt -\sqrt{a}$ or $x\gt \sqrt{a}$
Therefore:
$x^{2}\gt 16$
$x\lt-\sqrt{16}$ or $x\gt\sqrt{16}$
$x\lt-4$ or $x\gt 4$