Note that the problem states that you only need to check the solution she got for x...so we don't have to worry about y in this case. (YAY)
To verify MaryJo's answer, simply solve the equation for x:
3x - 2 = -x - 6
3x (+ x) - 2 = -x (+ x) - 6
4x - 2 = -6
4x - 2 (+ 2) = -6 (+ 2)
4x = -4
4x/4 = -4/4
x = -1
Since this conforms to MaryJo's answer, she did indeed find the correct solution for x.