MathNet 2022-part-05 — 197 problems (EN–VI) Question 16

The decreasing linear function $f(x)=k \cdot x+n$ satisfies $f(x+y)=f(x)+f(y)-3$ and $f(k)=2 f(1)$, for all $x, y \in \mathbb{R}$. What is the value of $f(-1)$?

(A) 4
(B) 0
(C) 2
(D) -2
(E) -4