coordinate geocircle

if the length of the tangent from Q (2,-1)to the point of contact P to the circle s: x(squared)+ y(squared)-2x-4y+k= 0 is 5, find k i dont know what to do !i havethe centre which is 2,-1 and the distance from centre to q ( square root of 10)

AHickey— 10/05/16