If you're saying that: 12g-19 = 3g-31 then,
Swap the sides around, getting common factors all on one
side.
E.G.
12g-3g = -31+19
9g = -12
g = -12/9
g = -1.333... or -4/3
Alternatively:
12g-19 = 3g-31
9g-19 = -31
9g = -12
g = -12/9
g = -1.333... or -4/3