Forget the derivative, you don't need it.

Suppose that the equation of the tangent line is

Substitute this into your given equation and solve for

If the line is to be a tangent this has to give you equal roots, (so

It will also give you two values of

Incidentally, to be on the safe side, you should also sketch the graph of your original function, (it's a circle centre (2,0) radius 1), to confirm that your results make sense.