Substitute

X=3y

X-3y=0

Answer:3y-3y=0

Step-by-step explanation: They are giving you a value for X that is 3y. You put that value in place of x. So, if x=12 and they want you to substitute for x-3y=0, you would have 12-3y=0 ... but the value of x in your question is 3y ... so you replace the x in the equation with it's value ... which is given to you as 3y ... or in my example, 12.

Step-by-step explanation:

x=3y

put

x=3y

3y-3y=0

0=0