The first one. is the right answer
False. You can find the solution more quickly by solving the system algebraically.
-4x + 2y = 18
Since y=-3x+4,
-4x + 2(-3x+4) = 18
-4x - 6x + 8 = 18
-10x = 10
x = -1
y = -3x+4 = 7
Divide AB from both sides to isolate the C
D = ABC
D/AB = ABC/AB
C = D/AB
C = D/AB is your answer
hope this helps