First of all, be sure you know that f(x) does not mean "f times x"
even though in basic algebra that's what it would have meant.  But 
in more advanced algebra it means something else: 

f(  )  says:

"Find the equation that starts off "f(x) =" and substitute 
whatever is in my parentheses for x in whatever is on the
right side of the equal sign in that equation."

-----------------------------------------
 
f(-2)  says:

"Find the equation that starts off "f(x) =" and substitute 
what is in my parentheses, which is (-2), for x in whatever 
is on the right side of the equal sign in that equation and
simplify."

So we find the equation that starts off "f(x)=" which is

f(x) = 5x+2

and we substitute (-2) for x in what is on the right
side, which is 5x+2


f(-4) = 5(-4)+2
f(-4) = -20+2
f(-4) = -18

-----------------------------------------
 
f()  says:

"Find the equation that starts off "f(x) =" and substitute 
what is in my parentheses, which is (), for x in whatever 
is on the right side of the equal sign in that equation and
simplify."

So we find the equation that starts off "f(x)=" which is

f(x) = 5x+2

and we substitute () for x in what is on the right
side, which is 5x+2

f() = 5()+2
f(-4) = +2
f(-4) = 
f(-4) = 

You do the other two the same way.

Answers: 

f(3) = 17
f(7) = 37

Edwin