Since the first equation gives you an expression for y, it is convenient to use that in the second equation. Since the second equation has a factor of 3 that can be removed from all terms, it may make the math easier to do that.
Revised 2nd equation: .. x -y = 1 Substituting for y: .. x -(2x +4) = 1 . . . . . use the expressiion for y in place of y .. -x -4 = 1 . . . . . . . . . simplify .. -4 = 1 +x . . . . . . . . . add x .. -5 = x . . . . . . . . . . . subtract 1
