Question

12=y-3y what's the answer

Answer 1

12=y-3y  Combine like terms
12=-2y  Divide
y=-6

Answer 2

12=y-3y

the same    y - 3y = 12 

-2y = 12   divide by 2 both sides (2 is a common factor for 2 and 12)

- y = 6 (multiply with -1)

y = - 6