Hello!
15w + 5w - 15w + 12 = -18 <=>
<=> 20w - 15w + 12 = -18 <=>
<=> 5w + 12 = -18 <=>
<=> 5w = -18 - 12 <=>
<=> 5w = -30 <=>
<=> w = -30 : 5 <=>
<=> w = -6
Good luck! :)
