I already know how to find the matrix of reflection over the $x$-axis (in the plane) (ON-base). By looking at what happens to the standard basis vectors $\hat{e}_1 = (1,0)$ and $\hat{e}_2 = (0,1)$. The matrix for the transformation will be: $$\begin{pmatrix} 1 & 0 \\ 0 & -1 \end{pmatrix}$$
That's an easy example. But I wan't to find the matrix for the reflection over a line that goes through the origin and makes the angle $\pi/17$ with the x-axis.
How can I approach this problem?