First, when b is below the y axis, it is obvious that b=-3.

Let g(0, b) be the distance on the parabola in direction B.

Y = x 2 and x 2+(y-b) 2 = 3 2 are simultaneous. If x is eliminated, a quadratic equation with one variable about y is obtained, so that the discriminant is equal to 0 and a solution is obtained.