Hey there!
Whenever you have a given slope and you're looking for the slope of a perpendicular line, always remember to use the opposite of the reciprocal.
Therefore, we know that the reciprocal of 3 (3/1) is 1/3.
The opposite is -1/3 - so that's the slope of the perpendicular line.
Additionally, to check if you got it right, multiplying the original and perpendicular slope should always get you -1.
Hope this helps!