Question

It solves the systems of equations.

Answer 1

3. Reduction

For example,
11x - 3y = 69
-3x + 3y = 3
___________
8x + 0y = 72 => x = 72/8 => x = 9 => -27 + 3y = 3 => 3y = 30 => y = 30/3 => y = 10.