(2)'s wrong.. in (1) you assumed that (x1,y1) is on the circle, so you can't use it as a point on the hyperbola.
you should solve (1) and x^2 - 2y^2 = 4 simultaneously and set the discriminant to 0.
Actually.. the better way:
Here's what I tried: Equation of tangent to circle: x[x1] + y[y1] =...