2x + 3y = 1
y = 3x + 15
Get the same variables on each side
3y = 1 - 2x
y = 15 + 3x
Multiply both sides of that second equation by 3 so we can subtract.
3y = 1 - 2x
3y = 45 + 9x
Subtract the equations from each other.
0 = -44 - 11x
Solve.
44 = -11x
-4 = x
Use x in an earlier equation to find y.
y = 3x + 15
y = 3 * -4 +15
y = -12 + 15
y = 3
x = -4, y = 3
