To change 3/4 to twelfths is a pretty simple thing to do, although I can understand your confusion. Basically, you just want to multiply the denominator, which is 4, until you have 12. So, to get 12, you will multipl 3 times 4.
3 3 9
/ * = /
4 3 12
So, basically, you are multiplying the numerator and denominator by the number you multiplied the to get to 12, so your final answer is 9/12.
An even easier way than this is to know your times tables! if you know them, you will easily be able to comprehend what the fractions are asking of you and you will know it immediately!
