-5/1. We know that the slope of a line (p) is perpendicular to that of line q, and so its slope will be the negative reciprocal. We can imagine this, as a line at 90 degrees to a line will have an opposite slope (positive/negative). By rotating it 90 degrees we also know that the run will become rise, and rise will become run, causing the reciprocal (keeping in mind that gradient is calculated by rise over run).
