Substitute x = 2 and x = 1/2 and solve for f(2).
Note: F(x) and f(x) are NOT the same function! Please be careful of "case" when you write these.
Let me expand a bit on TD's hint:
f(1/x) - 3f(x) = x
Let x = 2:
f(1/2) - 3f(2) = 2
Let x = 1/2
f(2) - 3f(1/2) = 1/2
Call f(1/2) = a and f(2) = b. Then we have the system of equations:
a - 3b = 2
-3a + b = 1/2
You want to solve this system of equations for b.
-Dan