Because [tex]x^2-c=108[/tex] can be rewritten as [tex]x^2-c-108=0[/tex], finding the value of c so that x=(+-)12 solves this equation it means to find the value of c so that [tex](+-)12^2-c-108=0[/tex].
Because [tex]12^2=(-12)^2=144[/tex], we must find c so that [tex]144-c-108=0[/tex].
Because [tex]144-108=36[/tex], c=36.
To check, replace c=36 in the initial equation, and check it is a solution for x=(+-)12.
