Multiply the denominator of the fraction part of the mixed number with the integer part. Then, add this amount to the numerator of the fraction to make an improper fraction. Do this for each mixed number involved in the problem. For instance, with 4 5/4 - 2 2/3, you would multiply 4 and 4 and add this to 5 to get 21/4. You would also multiply 3 with 2 and add it to 2 to make 8/3.
Determine what the least common denominator (smallest number both denominators can go into) is. Here, the common denominator is 12. Write a new blank problem underneath the original with the LCD for each denominator, including the answer. For example, you would write ?/12 - ?/12 = ?/12.
Figure out what you need to multiply each denominator of the original with to make the LCD. Then, multiply the corresponding numerator by the same number. For instance, 21/4 - 8/3 changes to 63/12 - 32/12.
Subtract the second numerator from the first and write the result over the LCD in the answer. When performing 63/12 - 32/12, you would get 31/12.
Simplify the answer if possible by dividing any shared factors out of both the numerator and denominator.