g = -21
it’s basic algebra, you get rid of the 9 by adding it to both sides. leaving you with
-g + 6 + .5g = 16.5
then you get rid of the 6, by subtracting it from both sides. leaving you with
-g + .5g = 10.5
combine like terms, which leaves you with
-.5g = 10.5
then you need to get g alone, so you divide by -.5 on both sides leaving you with
g = -21 :)