you cannot take (which you've done in 3rd step) just expand and simplify
So from the 3rd line, I went:
(10x^2 + 9000) - (30x^2 + 1000) = 0
-20x^2 + 6000 = 0
6000 = -20x^2
Then I took the square root of both sides, then divided to square root of 6000 by 20, and wualah. x=3.87
Edit: By the way, how do you get the math symbols?