R(x) = xp(x) = x(500 - 0.5x)
= -0.5x2 + 500x
The graph of R(x) is a parabola opening downward (since the coefficient of x2 is negative.
If A is negative, then the maximum of y = Ax2 + Bx + C occurs when x = -B/(2A).
So, the maximum of R(x) occurs at
x = -500/[2(-0.5)] = -500/ (-1) = 500
Maximum revenue = R(500) = -0.5(500)2 + 500(500)
= $125,000