first thing is to find the slope
slope = rise / run
m = (y2 - y1) / (x2 - x1)
m = (-1 - 1) / (5 - 4)
m = (-2) / 1
m = -2
you have a slope (m = -2) and a point (4,1) (either point works), so you can do point-slope form:
y - y1 = m (x - x1)
y - 1 = -2 (x - 4)
y - 1 = -2x + 8
y - 1 = -2x + 9 (slope-intercept form)