For part (a), I did , and I got . I then used the quadratic formula to get , giving or (because if touches , then ). However, the next step I'm confused about. Please help. What value of do I choose?

However, the next step I'm confused about. Please help. What value of do I choose?

The equation is the same as only if lies in . If you solve the two equations for with and you'll get a solution and another one : the right value of is the one such that .