x1^2+y1^2=16
x2^2+y2^2=16
(x1+x2)(x1-x2)+(y1+y2)(y1-y2)=0
4(x1-x2)-2(y1-y2)=0
4(x1-x2)=2(y1-y2)
(y1-y2)/(x1-x2)=k=2
y=2x+b
-1=4+b
b=-5
y=2x-5