Zina solves a system of linear equations by elimination and finds a solution of (2, 2). one of the equations is a + b = 4. what is the other equation?

The answer is -a + b = 0

If she wants to solve a system of linear equations by elimination and if one equation is unknown, one of the solutions in the unknown equation must be negative:
Known equation:       a + b = 4
Unknown equation: -a + b = ?

We know that a = 2 and be = 2, thus: 
Unknown equation: -2 + 2 = 0
The general form of the equation is -a + b = 0

Let's check it out:
Known equation:       a + b = 4
Unknown equation: -a + b = 0

Add them up:            2b = 4
                                   b = 4/2 = 2

a + b = 4
a = 4 - b
a = 4 - 2
a = 2

So, the second equation is correct.