To find the other system, since we know already that x = 3 and y = 9, we just have to input the value of x and y in each of the equations and see if they work.
4x + 6y = 64
4 (3) + 6(9) = 64
12 + 54 = 64
66 = 64
That doesn't work, so we move on.
2x + y = 36
2 (3) + 9 = 36
6 + 9 = 36
15 = 36
Once again, we move on.
4x + y = 21
4 (3) + 9 = 21
12 + 9 = 21
21 = 21
Since we're only looking for one answer, we have found it, meaning that the answer to this question is 4x + y = 21.
