Line n passes through points (7, 12) and (4, 15). what is the slope of a line that
is perpendicular to line n?

So anytime we have a slope and want to find the slope of a perpendicular line, we just have to remember a perpendicular line's slope is the negative reciprocal (reciprocal means flip the fraction). So if we had a slope of 3/4, the slope perpendicular to it would be - 4/3.

First we need to find our slope, to do this we take the change in y over the change in x like so:

(15-12)/(4-7) = - 3/4

The slope perpendicular to this is 4/3.