amaths

In the figure,the circle S:x^2+y^2-8x-4y+16=0,with centre K,touches the x-axis.The line L:y=mx,where m is not equal to 0,passes through the origin and cuts the circle S at two points A(x1,y1) and B(x2,y2).M is the mid-point of AB
(a)By substituting y=mx into the equation of S,show that
x1+x2=[4(m+2)]/(1+m^2)
(b)If the coordinates of M are (3,p),find the value of p.Also find the length of AB

http://i214.photobucket.com/albums/cc72/yesterday0721/16.jpg