Do you not understand the definition of linearity?
You need to compute f(ax) and af(x) and say wow they really are equal.
Then you compute f(x+y) and f(x) + f(y) and again note that they are equal.
OR, you can compute f(ax+by) and af(x) + bf(y) and note that they are equal.
There are smoother ways of doing this but what I outlined above is fine for a beginning proof student.
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