This video tutorial shows you how to multiply mixed numbers. When doing this the first thing that you want to do is put the mixed numbers into a improper fraction form. From this step you can simplify the numbers based on what you are using. In this case you can take out the 3 and 21 and simplify it to 1 and 7 respectively. From here you do the basic multiplication to get one improper fraction. Then turn this number back into a mixed fraction to complete your problem.