I am really stuck on this question...

I have my Maths Prelim in a couple of days and I think I am ok with most things however I am stumped by this one question.

The line y-2x=k is tangent to the circle x^2+y^2-2x-4=0, where k>0.

Find the value of k.

I have got to x^2+(k-2x)^2-2x-4=0, I know that I need to apply that to the discriminant (b^2-4ac) but I don't know how to get that into the discriminant.

Thanks For Any help,

Bumfluff