y−3x=13y−3x=13 for yy.
I'm not sure this is the solution you are expecting....
you have
y-3x=13y-3x if you add 3x to both sides you get
y = 13y which only works for y=0
then 13y - 3x = 13
13(0) - 3x =13 yields x = -13/3