Do you know what f(4) and f(6) means?  Apparently not.

f(4) means what you get when you substitute (4) for x in the right side of 

f(x) = 3x-12

If you put a (4) wherever the x is you get

f(4) = 3(4)-12 and if you simplify the right side you get

f(4) = 12-12

or

f(4) = 0

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Similarly, f(6) means what you get when you plug 6 in for x in
the right side of 

f(x) = 3x-12

If you put a (6) wherever the x is you get

f(6) = 3(6)-12 and if you simplify the right side you get

f(6) = 18-12

or

f(6) = 6

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Now if you tried doing those two substitutions in any of the other

choices you had on your test you would not get 0 for f(4) and also 

6 for f(6).

Learn that "f( )" with a number in the parentheses means 

to substitute whatever number is in the parentheses for x into

the right side of the equation which has "f(x) = ____" where the 

right side is some algebraic expression that contains x, such as

3x-12 or 6x-3 or 7x+3 or 8x²+3x-9 or etc. etc. etc.  Then after

substituting, then  simplify, and the number you get is equal 

to f(that number)

Edwin