How to find $x$ from expression? 
Solve for $x$ in the expression $$x(2x + 5) = 168.$$

I have already tried to move $x$ from one side to another with brackets and without them, like this:
$x = 168;\quad 2x + 5 = 168 \implies 2x = 163 \implies x = 81.5$
but eventually without success. 
I know that the answer is $x = 8$ but I can't come to this solution.
How to solve this?

That's a quadratic equation ($ax^2+bx+c=0$) because $x$ there occurs to a second power. Multiply each term in the parentheses by $x$ and move $168$ over to the left side:
$$
x(2x + 5) = 168\implies\\
2x^2 + 5x - 168=0.
$$
Now, use the quadratic formula to find the roots (the solutions to your equation):
$$
x_{1,2}=\frac{-b\pm\sqrt{b^2-4ac}}{2a}.
$$
Your $a$ there is $2$, $b$ is $5$ and $c$ is $-168$. You will see that $-10.5$ is another solution to your equation.
