We must complete the square in order to solve the problem.
Now, the first step is to realize that if we have factor a quadratic (x+a)^2
[tex](x+a)^2=x^2+2ax+a^2^{}[/tex]which, in our case, implies
[tex]2ax=36x[/tex][tex]\therefore a=18[/tex]Therefore, we add 18^2 to both sides of the equation such that
[tex]x^2+36x\textcolor{#FF7968}{+18^2}=(x+18)^2[/tex]this gives
[tex]x^2+36x+18^2=88+18^2[/tex][tex]\textcolor{#FF7968}{\Rightarrow(x+18)^2=412}[/tex]which is choice c from the options.