According to the slope formula, if we have two points (a, b) and (c, d), the line which passes through them will be (y - b) / (x - a) = (d - b) / (c - a).
Thus, using the slope formula, we have:
(y - 1) / (x - 4) = (-9 - 1) / (2 - 4)
(y - 1) / (x - 4) = -10 / -2
(y - 1) / (x - 4) = 5
y - 1 = 5(x - 4)
y - 1 = 5x - 20
y = 5x - 20 + 1
y = 5x - 19, which is the equation you need.