$\displaystyle x^2+y^2=4x$

The tangents at the points P and Q on this circle touch the circle $\displaystyle x^2+y^2=1$ at the points R and S. Find the coordinates of the point of intersection of these tangents, and obtain the equation of the circle through the points P, Q, R, and S.

I need to know how to find the equation of the common tangent, but I don't know where to start.

Thanks