Having a bit of trouble with this one. Can anyone help me out?

Many thanks.C is a circle with centre (-1, -4). The midpoint of a chord of length is (2, 0). Find the length of the radius of C.

Q.

Attempt:Perpendicular distance [bc] from centre c to x-axis is 4,

Let |ab| = 1/2 length of chord on x-axis = ,

radius,

(From text book): Radius =

Ans: