In this case, you have to find a common multiple of 3a^2 and 4ab. As only one of the numbers is divisible by two, this means that two cannot go outside of the bracket. The only other aspect that is in both sides of the equation is a, therefore, this goes outside the bracket. The best way to approach this is to divide both sides by a, and this will give you what is inside the bracket. 3a^2 divided by a is 3a, therefore, this is the first aspect in the bracket. -4ab divided by a, leaves -4b. Therefore, these go in the bracket.
3a^2- 4ab simplified is a(3a-4b)
Hope this helps