−g + 6 + ½ g – 9 = 7 ½

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 :)