Turn them into improper fractions. So, 21/4 - 13/9. Then give them common denominators; for this problem, just use their least common multiple; 36. So, 189/36 - 52/36. So you get, 137/36. Simplify, 3 29/36.
First you turn both fractions into improper fractions Next multiply 9/9 times 21/4 Then multiply 4/4 times 13/9 Now you have common denominators Next find the answer to the problem Finally , simplify your answer