(50 points) which equation combines with the given equation to form a system of equations with the solution x = 3 and y = 9? x + 2y = 21

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. 

C
If you plug in the values in that equation, it works.