Question

If 3x = 12, then x = 4.

Answer 1

Correct, because 3 multiplied by 4 is 12. 12=12 

Answer 2

If the question says that x=4, then we can plug in 4 for every x we see.

So: 3×4.  THree times 4 does equal twelve, so the equation is correct.  So you're right in saying x=4