Question 804175
(1,2) and (-1,-2)

The equation when two points are given 

{{{(y-y1)/(y1-y2) = (x-x1)/(x1-x2)}}}


where (x1,y1) (x2,y2) are the two points 

{{{(y-2)/(2-(-2)) = (x-1)/(1-(-1))}}}


{{{(y-2)/4 = (x-1)/2}}}


y-2=2(x-1)

y-2=2x-2

y-2x=0