I am assuming that x on the left side is f(x) or y. We must find the vertex of the parabola (graph of this function). First, find all the lowest x value possible, then plug it in the equation. The formula for this is -2b/a
x =-2(0)/-1
x = 0

Plug in x = 0, and you get y = 4
Since the sign of x^2 is negative, the parabola opens downward.

The range of this function is {y|y <= 4}