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?

if $f(x)g(x) = A$ that does not mean that $f(x)=A$ or $g(x) = A$. However, what you could do is expand as follows
$$2x^2 + 5x = 168$$
then
$$2x^2 + 5x - 168 = 0$$
then you have to realize that what you have is a quadratic equation, where normally you compute a quantity called the discriminant
$$\Delta = b^2 - 4ac$$
for a quadratic equation $ax^2 + bx + c $. In your case, you got $a = 2,b=5,c=-168$ so
$$\Delta = (5)^2 - 4(2)(-168) = 1369$$
Now since $\Delta > 0$, you have two distinct solutions,
$$x_1 = \frac{-b - \sqrt{\Delta}}{2a}=\frac{-5 - \sqrt{1369}}{4}$$
$$x_2 = \frac{-b + \sqrt{\Delta}}{2a}=\frac{-5 + \sqrt{1369}}{4}$$
