What is the matrix used to reflect a point across an arbitrary line in the coordinate plane?

In geometry class, I studied transformational geometry. In my textbook, there is a graphing calculator activity where you imput matrices to transform objects. So I was fascinated and began to learn matrices on my own. I have matrices for arbitrary rotation, dilation, and translation, but none for reflection. So I researched it, but found nothing. Is there a matrix for reflection ? If so, what are the entries ? If not, explain why.