Persamaan garis yang melalui titik (2,1) dan titik (-2,-7) adalah

(y-y1)/(y2-y1) = (x-x1)/(x2-x1)
(y-1) / (-7-1) = (x-2) / (-2-2)
(y-1) / -8 = (x-2) / -4
-8(x-2) = -4(y-1)
-8x + 16 = -4y + 4
-8x + 4y + 16 - 4 = 0
-8x + 4y + 12 = 0
=> -2x + y + 3 = 0

Y-y1/y2-y1=x-x1/x2-x1
Y-1/-8=x-2/-4
-8(x-2)=-4(y-1)
-8x+16=-4y+4
4y=8x-12
Y=2x-3