A line with a negative slope is perpendicular to one of the lines shown. What is its slope? Use the graph shown below.

A line with negative slope is always perpendicular to a line with positive slope, since perpendicular slopes (of non horizontal and non vertical lines) are opposite signs and reciprocals.

So, you have to be perpendicular to the blue line, since that's the one with a positive slope.  Since the slope of the blue line is "up 1, right 3" or 1/3 (found by getting from the y-intercept to the next point to the right) your line's slope is the opposite sign and reciprocal.

That makes your slope positive -3/1 or -3.