For every point (x,y) on the graph
y = 30 - 5x^2, consider a rectangle with corners (-x, 0), (-x,y), (x,0), and (x,y).
For which value of x does the rectangle have a maximum area?
A rectangle has maximum area when it is a square.
That is, when the base (2x) is equal to the height (y)
30 - 5x^2 = 2x
x = 2.257