Answer
The relation $x=y^2$ is not a function as some x-values are paired with more than one y-value.
Work Step by Step
RECALL:
$\\\\$A relation is a function if each x-coordinate is paired with only one y-coordinate.
$\\\\$In the given relation, the points (4, -2) and (4, 2) satisfy the given equation.
$\\\\$This means that the x-value of $4$ is paired with two different y-values.
$\\\\$Thus, the relation $x=y^2$ is not a function.