Ineed the a recipe calls for 3/4 cup of vegetable broth. you have 2/3 cup of vegetable broth. how much additional broth is need for the recipe

You need 1/12 cups of additional broth. To get this answer, you first need to find the least common multiple of the denominators. So, the LMC of 3 and 4 is 12. Multiply the top and bottom of the fractions so that they share the same denominator. Multiplying 3/4 with a 3 on both the numerator and denominator gets you 9/12 while multiplying 2/3 with a 4 on both the top and bottom gets you 8/12. After that, subtract 8/12 from 9/12 to get 1/12.