The perdendicular slope of the line stated would be 1/3.
Perpendicular slopes are the opposite sign of the slope stated (positive turns to negative and vice versa) and whole numbers change to fractions (and vice versa). Since the stated slope is negative, it turns to a positive, and since it's a whole number, it changes to a fraction. Thus making 1/3 the perpendicular slope.
Here's an image of a graph to show this.
I hope this helps!