Motivated by Sy's last question, here is a fun problem for those with some linear algebra knowledge: $1. If $T:X\rightarrow Y$ is a linear map between vector spaces, and for some $q\in Y$, $p\in X$ is a solution to the equation $Tx=q$, prove that the general solution is given by $x=p+r$ where $r\in \ker(T)$.\\ \\ 2. What is the general solution of the equation $\left(\frac{d}{dx}-\lambda\right)u=0$ where $\lambda\in \mathbb{C}$ and $u\in \mathcal{C}^\infty(\mathbb{R})$?\\ \\ 3. Explain why the general solution to a linear ordinary differential equation with constant coefficients is a subspace of $\mathcal{C}^\infty(\mathbb{R})$ of dimension $n$. Find a basis for this subspace. $