1. The problem states that the function $u(x_1, x_2) = 2 x_2$. 2. This means that the value of $u$ depends only on the variable $x_2$ and is twice its value. 3. There are no additional operations or variables affecting $u$, so this expression is already simplified. Final answer: $$u(x_1, x_2) = 2 x_2$$