The slope of a line through points (x₁, y₁) and (x₂, y₂) is m = (y₂ - y₁) / (x₂ - x₁). This is the change in y divided by the change in x, or rise over run.
A line through (1, 2) and (3, 8) has slope (8 - 2) / (3 - 1) = 6 / 2 = 3. The line rises 3 units for every 1 unit it moves right.
Watch for vertical lines where x₂ = x₁. The slope is undefined (division by zero). Horizontal lines have slope 0.