Garis melaui titik (2,a) dan (-1,3) bergradien -2/3

x1 = 2
y1 = a
x2 = -1
y2 = 3

m = -2/3

m = y2-y1/x2 -x1
-2/3 = 3-a/-1-2
-2/3 = 3-a/-3
-2/3 x -3 = 3-a
2 = 3-a
2-3 = -a
-1 = -a
a = 1

Maaf kalau salah