I assume you want to solve for w. So, we will need to put w on one side and everything else another side.
-w + 4(w + 3) = -12 ; Our equation
-w + 4w + 12 = -12 ; Distribute the 4 across w and 3
3w + 12 = -12 ; Combine like terms, -w and 4w
3w = -24 ; Subtract 12 from each side of the equation
w = -8 ; Divide both sides by 3.