hi,
this problem is a simple substitution.
so you know that y = 15, so substitute it into the equation you were given!
$${\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{y}} = {\mathtt{30}}$$
$${\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{15}} = {\mathtt{30}}$$
so now with 15 substituted in, all you have to do is move the 15 over to the other side!
$${\mathtt{x}} = {\mathtt{30}}{\mathtt{\,-\,}}{\mathtt{15}}$$
and now simplify by subtracting 15 from 30!
$${\mathtt{x}} = {\mathtt{15}}$$
hope this helps!