Answer: B. x, in the second equation
Step-by-step explanation:
The given system :

To apply the substitution method , first we need to select the best variable and a equation which is easy to simplify.
From the given system equation (2) have all the numbers which are multiples of 3.
Then we divide 3 on both sides of equation (2), we get

Therefore, the best variable to solve for and from what equation is x, in the second equation.
Answer: 18
Step-by-step explanation: 6 x 3 = 18, 18 x 1 = 18
I think A is the answer I might be wrong tho