This is about multiplying fractions. I got the easy one just

This is about multiplying fractions. I got the easy one just fine. I don’t understand the harder one as an example that they give you in the book. It says 2 1/3 * 5 1/4 = 7/3 * 21/4 = 7*21/3*4 = 7*3*7/3*4 = 49/4 = 12 1/4. What I don’t understand about this problem is why there is an extra seven in the 7*3*7. I know that we are trying to find the factors for 21, but I thought you were only supposed to use the lowest ones and I thought that would be 7*3?