Suppose that $\mathcal{U}$ is a subspace of $\mathbb{R}^3$.

Also suppose that the projection of $\mathbf{x}$ onto $\mathcal{U}$ is $(1,1,0)$ and the projection of $\mathbf{y}$ onto $\mathcal{U}$ is $(1,1,-1)$.

What is the projection of $\mathbf{x} + \mathbf{y}$ onto $\mathcal{U}$?