First of all, you can sum the two terms involving g on the left hand side:
[tex] -6g + 3g + 12 > -18 \iff -3g + 12 > -18 [/tex]
Subtract 12 from both sides:
[tex] -3g > -30 [/tex]
Divide both sides by -3: note that since we're dividing by a negative number, we must invert the inequality sign:
[tex] g< 10 [/tex]
