3x + y = 3
7x + 2y = 1
First isolate one of the variables (x or y) in one of the equations.
Isolate "y" in the first equation(because it is the easiest to isolate) and substitute it into the second equation.
3x + y = 3 Subtract 3x on both sides
3x - 3x + y = 3 - 3x
y = 3 - 3x
7x + 2y = 1
7x + 2(3 - 3x) = 1 [since y = 3 - 3x, you can substitute (3-3x) for "y"]
Multiply/distribute 2 into (3 - 3x)
7x + (3(2) - 3x(2)) = 1
7x + 6 - 6x = 1
x + 6 = 1 Subtract 6 on both sides
x = -5
Now that you know "x", substitute it into one of the equations (I will do both)
3x + y = 3
3(-5) + y = 3 [since x = -5, you can plug in -5 for "x"]
-15 + y = 3 Add 15 on both sides
y = 18
7x + 2y = 1
7(-5) + 2y = 1
-35 + 2y = 1 Add 35 on both sides
2y = 36 Divide 2 on both sides
y = 18
x = -5, y = 18 or (-5, 18)