jmc

Let's write the first sentence as an equation: $$\begin{array}{rl}\frac{x}{y}& =\frac{1}{2},\\ 2x& =y.\end{array}$$

Now, we can substitute this into the given equation to find $x$: $$\begin{array}{rl}2x& =4x-36,\\ 36& =2x,\\ \boxed{18}& =x.\end{array}$$