G+2(3+g)=-4(g+1) can't figure out need

You just need to solve for g, multiply it out to get -g+6+2g=-4g-4, subtract 6 from both sides to get -g+2g=-4g-10, then add 4g to both sides for -g+2g-4g=-10, then add the g's together to get -3g=-10, divide both sides by -3 and you get g=3,1,3 (three and one third).

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.