From two points: slope, distance, midpoint and the line equation y = mx + b.
Slope m = (y2 - y1) / (x2 - x1), the rise over the run between the two points. A vertical line (x1 = x2) has an undefined slope, which the tool reports.
The straight-line distance between the points, their midpoint, and the full line equation in slope-intercept form y = mx + b.
Yes, all four coordinates accept negatives and decimals.