How to show it is Linear? (urgent!)

[lo2]

I have this Linear Transformation \(\displaystyle f: P_3(R) \rightarrow R^2\) given by

\(\displaystyle f(P(x))=(P(1),P(-2))\)

Then I have to show that \(\displaystyle f\) is linear.

I do not really have any idea... SO really need some help please!