How do I add 2/3 and 3/4 when the denominators are different?

When denominators differ, you must make them the same before adding. First find the LCM of the denominators 3 and 4, which is 12. Now convert each fraction to an equivalent one with denominator 12. For 2/3, multiply top and bottom by 4 to get 8/12. For 3/4, multiply top and bottom by 3 to get 9/12. Now the denominators match, so add only the numerators: 8/12 + 9/12 = 17/12. Since 17 is bigger than 12, you can write it as the mixed number 1 and 5/12. Always check if the final fraction can be simplified; here 17/12 is already in lowest terms.