You try to get all the numbers on one side of equal sign, and letters on other side. First you must multiple the part in brackets { 1/2(g+6) is equal to 1/2g +1/2(6) which equals 1/2(g)+3 ...

now subtract 3 from each side of the equal sign 1/2g+3-3=-2g-3 then add 2g to each side of the equal sign 1/2g +2g = -3