Hey Bird:)

When you see these type questions with f(x), (called "functions"), you put in whatever is in the f(->here<-:) in for x. So, f(x) = -3x - 2, you could do "Find f(2)."  Then, you plug in "2" wherever you see "x". 
So, for f(x) = -3x - 2:

f(2) = -3(2) - 2

= -6 - 2

= -8 

Following me so far?

Then, f(x) = -3x - 2:  Find f(d-7).

f(d-7) = -3(d-7) - 2

= -3d + 7 - 2

= -3d

= 5.

Do you understand that?  Hope so!