For this question you need to turn the mixed number to a improper fraction. Which is 21/6+(-10/7). Then find the least common factor which is 42. Then think what multiplies 7 to get 42 which you need to think with 6 which is 6 for 7 and 7 for 6. Then multiply 7 to 21/6 on the numerator and the denominator same with the other fraction. Then add the fractions and then turn the improper fraction to a mixed number again which is your answer.
Answer: 18 11/42