A line with positive slope is always perpendicular to a line with negative slope, since perpendicular slopes (of non horizontal and non vertical lines) are opposite signs and reciprocals.
So, you have to be perpendicular to the red line, since that's the one with a negative slope. Since the slope of the red line is "down 5, right 1" or -5/1 (found by getting from the y-intercept to the x-intercept), your line's slope is the opposite sign and reciprocal.
That makes your slope positive 1/5.
