It is the negative reciprocal to the slope of the line it is perpendicular to. For example, if the slope of that line was 3, the negative reciprocal would be -1/3, because if you multiplied them, -1/3 × 3/1, you'd get 3/3 which is 1. If the slope given was -4, the negative reciprocal would be 1/4, the negative reciprocal given of 1/2 would be -2, etc. I hope this helps! Let me know f I've just confused you :)
