We have sussed it out using half your answer and half sonic booms answers

No substitute x = 8 into original equation

Y = (5000 * 8) – (625 * 8)

Y = 40000 – 5000

Y = 35000

hope this helps

You forgot to square the 625 8x ^2

so

40000 - 625 x 64 =y

y = 40000 - 40000 =0

So are we correct in saying

x = 8

y = 0