How I have to proceed here?
5f(-1)-3=
5f(-1)=3
5f=3/-1
f=-3/5
or
5f(-4)=
5f=1/-4
f=-0,25/5
5f(3)=
15f=
f=1/15
You question is a bit confusing.
If f is just a normal variable then
5f(-1)-3
=5*f*-1 -3
=-5f-3
However, if f(-1) means function of -1 then you cannot simplify it without more information.
+8 
