Learn (or just recall) how to calculate the slope of a line, the slopes of parallel and perpendicular lines, and if you're really lucky, the slope of a line tangent to a circle. Hope that you are really lucky, since that's the most fun you can have with slope.

Geometry 2(B) derive and use the distance, slope, and midpoint formulas to verify geometric relationships, including congruence of segments and parallelism or perpendicularity of pairs of lines