Y = 5000x – 625xsquared

First dy/dx = 0
So divide both side by x

y/x = (5000x / x) – (625x2/x) = 0

cancel the x’s out
0 = 5000 – 625x

Divide all parts by 625
0/625 = (5000/625) – (625x/625)

simplify
0 = 8 – x

Plus x to both sides

X = 8

No substitute x = 8 into original equation

Y = (5000 * 8) – (625 * 8)

Y = 40000 – 5000

Y = 35000

hope this helps