Here is a site that clearly and thoroughly explains many topics surrounding the slope of a line, including slope of horizontal, vertical, parallel, and perpendicular lines in addition to the formula and concept of slope. There are...

Twenty-two problems present a practice finding the slope of a line. The solutions are only available with a paid subscription.

A tutorial on slope with many step-by-step examples. The tutorial clarifies what slope is, how to find slope between two points on a graph, and how slopes of parallel and perpendicular lines are related.

This tutorial will show you how to find a the slope of graph, write a linear equation in slope-intercept form. You'll also learn how to determine whether two lines are parallel or perpendicular to each other. For advanced math students.

Use your knowledge about the slopes of parallel and perpendicular lines to solve some problems. For example, what's the equation of the lineperpendicular to y=3x-3 and passes through the point (-8, -2).