Okay, so firstly we need to find an input to substitute for n to take advantage of the information we already have. If we let n=3 and plug in, we get
f(3)=3f(1) - 2f(2)
From that value, we can plug in the information we had into the equation, giving us
f(3)=3(-1) - 2(3) => f(3)= -9
Then, we repeat these steps. We now know the values of f(3), f(2), and f(1). Can you take it from there?
All the best.
BasicMaths