An equation is an expression that involves a variable, and the goal is to find one (or more) value(s) for the variable such that the equation becomes an indentity (i.e. left and right hand sides are the same).
So, in order to check wether 6 is a solution to your equation, you have to replace every occurrence of "x" with "6". In this case, the variable appears only once, so the equation becomes
which is true, so 6 is indeed a solution