$$\sum_{i=1}^{x} x + \sum_{i=1}^{x} 2 = 8$$
Trying to discover $x$ here, so I made: $$x(x-1) + 2(x-1) = 8$$
Which should give me the $x$, but instead of an integer I get: $$x = \frac{-1\pm\sqrt{41}}{2}$$
When the answer should be $x=2$, according to the solution.
What am I doing wrong?