Be careful when subtracting mixed numbers. You may need to borrow from the whole. If you borrow one whole you need to convert that to a fraction to add to the fraction portion of the mixed number. You have not changed the value but have renamed the mixed number in a way that allows you to compute. In the second example pictured below, the one whole was converted into 27/27 to because it needed to be added to 3/27.
The video below shows the algorithm step by step.