D. Multiply both sides of the top equation by 3
Step-by-step explanation

to do that, both equations must have equal x-coeeficient, we need
Step 1
check the coefficients


Now, to go from C1 ( 4) to C2( 12) we can multiply the top eqution by 3 , this way we would get

or
to go from C2 ( 12) to C1 ( 3) we can divide the bottom equation by 3

therefore, the anwer is
D. Multiply both sides of the top equation by 3

now it has 12 as coefficient for x
I hope this helps you