It's a pretty long process, but you first have to find the 'negative reciprocal' of the slope of the line that it is perpendicular to. So if that slope is 3, the negative reciprocal would be -1/3, and if the slope was 1/2, the negative reciprocal would be -2, etc. So once you have this, you then have to substitute the x and y coordinates into y = mx + c to find the y-intercept, or 'c'. If your coordinates were (3,8) for example, yours write the equation like 8 (the y coordinate) = (3 × the negative reciprocal) + c and then you'd solve the equation. I hope this helps! If you have a question about perpendicular lines or if I made you more confused then please let me know :)
