The 3rd and 5th systems of equations are the answer.
Point A is at (3,-1), so all you do is input 3 for x and -1 for y into all the equations. For the systems to work, both equations must work with (3,-1).
Let the radius of the circle be r. Then the line from the external point through the center of the circle which extends to the far point on the circle has length 3r .By the tangent - secant theorem
t^2 = 3r * r = 3r^2 ( where t is the length of the tangent).
