After solving a system of linear equations how can you check

After solving a system of linear equations, how can you check your answer to determine whether it is correct?

You plug the values into the equation.

x + y = 7

3x - y = 9

Add the equations.

x + 3x + y - y = 7 + 9

4x = 16 --> x = 4

3(4) - y = 12 - y = 9 --> y = 12 - 9 = 3

x + y = 4 + 3 = 7