Let's break the phrase into to parts:
"...6 times a number f" this part is the multiplication of 6 by f, so: 6f.
"7 less than something" this means that the result will be whatever is int something but 7 less, so it is something minus 7. The something we already saw that is 6f, so, we have 6f - 7 as the answer. Alternative d.