As with a system of equations, call them A and B in order to keep them straight.
A: x + y = 7
B: x = y + 3
Because in B we have x on one on one side and not x on the other, substitution is the way to go. Substitute equation B into equation A.
x + y = 7 Equation A
y + 3 + y = 7 Putting B into A
2y + 3 = 7 Collect like terms
2y = 4 Subtract 3 on both sides
y = 2 divide both sides by 2
With y = 2 we can substitute that back into an original equation. Choose B.
x = y + 3
x = 2 + 3
x = 5
Thus x = 5 and y = 2, or (5, 2) is the solution of this system.
