how do you add mixed fractions?

discover the common denominator, then add the numerators. For example,

2 3/4 + 1 2/3 - 3 2/7
= 11/4 + 5/3 - 23/7

The common denominator is 4*3*7, so that makes things

= 11*3*7/84 + 5*4*7/84 - 23*4*3/84
= 231/84 + 140/84 - 276/84
= 95/84
= 1 11/84