tolong jelaskan cara soal nomor 2b

P (-1, 2) dan Q (1, 1)

m = (y2 - y1)/(x2 - x1)
m = (1 - 2)/(1 + 1)
m = -1/2

y - y1 = m(x - x1)
y - 2 = -1/2(x + 1)
-2(y - 2) = x + 1
-2y + 4 = x + 1
-x - 2y = 1 - 4
x + 2y = 3