To add fractions the denominators must be equal.

Find the least common denominator (LCD).

Write the prime factorization of each denominator.

Multiply the highest power of each factor in either number.

LCD of the fractions is 21.

Rewrite the equivalent fractions using the LCD.

Rewrite the expression using the LCD.

Denominators are same, write difference of numerators over denominator.

                  (Subtract the numerators)

The sum is .