To combine like terms, you combine all of the same variables.
For example, if you have 5x+7y+5x, you would combine like terms to make 10x+7x.
So for your first problem, we know that the combined like term is 15g. We know that one of the numbers combined was 3g. With that knowledge, you can subtract 15g-3g and get 12g.
I hope this makes sense, and helps you answer your other questions!
My recommendation would be to factor.. x² + 4x + 3 = 0 factor the left side into two binomials (x + 3)(x + 1) = 0 set each of these binomials equal to zero and solve