I found this in a book but I am not able to understand how they got this result. It goes the equation family of circles touching a given line $(y-y_1)=m(x-x_1)$ at $(x_1,y_1)$ for any value of $m$ is $(x-x_1)^2 + (y-y_1)^2 + \lambda[(y-y_1) - m(x-x_1)] = 0$ where $\lambda$ is the parameter
Any help would be appreciated