You could something like:
21g + 6h = 3(7g + 2h)
Here's why: When you have to create an equivalent expression, that means the formula on one side of the equal sign, has to be the same as the formula on the other side. So in a formula like mine, when you have a number on the outside of the parentheses, (in my formula, that's the number 3) you have to multiply it by all the numbers INSIDE the parentheses, (which in my formula is 7g and 2h) and when multiplied through, have to equal the same formula on the other side of the equal sign. (in my formula, when multiplied it equals 21g and 6h)
Hope that makes sense, and helps!