Sorry i did not have tiime to explain my attempt:
5x2 =x(2x+1)
10 = 2x^2 + x
0 = 2x^2 + x - 10
2 / 0 = 2 / (x^2 + x - 10)
0 = x^2 + x - 10
0 = (x -10) (x + 1)
(split into two equation)
x = 10 <---
x = -1

In the end im left with x = 10, when in my textbook x = 2 ?