What is the difference between dividing fractions less than 1 and dividing mixed numbers

Me. i can't figure out this question. give an explanation too. .

If f(x) = 4x - 3 and g(x) = x + 4, find (f - g) (x)

What will be the end result for the taxpayer who filed his federal income tax return using the 1040ez form shown belowa he'll owe $173b he'll receive a refund of $173c he'll owe $3921d he'll receive a refund of $3921