P(2, -8) and Q(1, y)
Gradient of slope(m) = $\;{y_2 - y_1}/{x_2 - x_1}$
- 4 = $\;{y - (-8))}/{1 - 2}$
-4 = $\;{y + 8}/{- 1}$
cross multiplying
4 = y + 8
y = 4 - 8 = -4
