Answer:
Multiply both sides by 3x + 7
Explanation:
The reason you must multiply first is because you generally want to stay away from binomial division, as it is much easier to work without fractions. Once you multiply both sides by 3x + 7, you effectively only have to work with numerators now, instead of both numerators and denominators. After you have shifted by multiplying both sides by 3x + 7, you can isolate x by adding and subtracting from both sides, but you generally what to stay away from dividing variables.
Hope this helps!