Question 85
Find all functions $f: \mathbb{R} \rightarrow \mathbb{R}$ such that
$$
2 f(x) = f(x + y) + f(x + 2y)
$$
for all $x \in \mathbb{R}$ and for all $y \geq 0$.
$$
2 f(x) = f(x + y) + f(x + 2y)
$$
for all $x \in \mathbb{R}$ and for all $y \geq 0$.