Combine like terms. do you combine -w and 4w? if so, explain why.

 Yes; you do combine  "-w" and "4w" ;  because they are both "like terms" ;  that is, they are both terms with the SAME first degree single variable in the entire expression.

So, to combine them:
 -w + 4w = -1w + 4w = 3w ;

and we can simplify the entire expression:

" -w + 4w + 12 " ; 

to:    " 3w + 12 " .

  Hope this explanation helps!