Easiest way to prove that an operator is linear? 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.

You simply verify that the conditions of linearity are satisfied: for every $a\in\Bbb R$ and $\vec u,\vec v\in\Bbb R^4$, $h(\vec u+\vec v)=h(\vec u)+h(\vec v)$, and $h(a\vec u)=ah(\vec u)$. I'll do the first and leave the second for you.
Let $\vec u=\langle u_1,u_2,u_3,u_4\rangle$ and $\vec v=\langle v_1,v_2,v_3,v_4\rangle$. Then $$\begin{align*}
h(\vec u+\vec y)&=h\Big(\langle u_1,u_2,u_3,u_4\rangle+\langle v_1,v_2,v_3,v_4\rangle\Big)\\
&=h\Big(\langle u_1+v_1,u_2+v_2,u_3+v_3,u_4+v_4\rangle\Big)\\
&=\Big\langle 2(u_1+v_q)+3(u_2+v_2)-(u_3+v_3)+2(u_4+v_4),\\
&\qquad\qquad(u_1+v_1)-5(u_2+v_2)+6(u_4+v_4),\\
&\qquad\qquad2(u_2+v_2)+(u_3+v_3)+(u_4+v_4)\Big\rangle\\
&=\Big\langle 2u_1+3u_2-u_3+2u_4,u_1-5u_2+6u_4,2u_2+u_3+u_4\Big\rangle\\
&\qquad\qquad+\Big\langle 2v_1+3v_2-v_3+2v_4,v_1-5v_2+6v_4,2v_2+v_3+v_4\Big\rangle\\
&=h\Big(\langle u_1,u_2,u_3,u_4\rangle\Big)+h\Big(\langle v_1,v_2,v_3,v_4\rangle\Big)\\
&=h(\vec u)+h(\vec v)\;.
\end{align*}$$
