Hi Cj313. In order to reduce this fraction to its lowest terms, we will have to divide both the numerator and denominator by the highest possible number without leaving any remainder. So, the number that both of them are divisible by is 12. If you divide 84 by 12, you will get 7. If you divide 144 by 12, you will get 12. So, your final answer is 7/12.
Hope it helps :)