I have an operator
$$h: \mathbb R^4 \to \mathbb R^3\text{ given by } h(x, y, u, v) = (2x + 3y - u + 2v, x - 5y + 6v, 2y + u + v)$$
What is the easiest way to proove that this operator is linear?
I looked over on wiki etc., but I didn't really find the way to prove it mathematically.