g + u = 676
6.5g + 8u = 4902.5
First, solve the top for g or u. Let's do u.
u = 676 - g
Now, since the two sides are equal, we can swap them out for each other.
6.5g + 8(676 - g) = 4902.5
6.5g + 5408 - 8g = 4902.5
-1.5g = -505.5
g = 337
u = 676 - 337
u = 339