Question 240663
Let the ? be represented by z. Now solve for z
{{{9/(x-4) = (9x+36)/z }}} Use distribution on the left side numerator
{{{9/(x-4) = (9*(x+4))/z }}} divide both sides by 9
{{{1/(x+4) = (x+4)/z}}} multiply both sides by z
{{{z/(x+4) = (x+4)}}}  isolate z
{{{z = (x+4)(x-4)}}} simplify
{{{z = x^2-16}}}