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