Question 329163
I(x) = (500 - x)(500 + 5x) become y = -5x^2 + 2000x + 250,000 after multiplying the left side of the original equation. 

Let y = 0 and solve for x.

Now, use the quadratic formula where a = -5, b = 2000 and c = 250,000 to solve for x.  Can you take it from here?