To calculate the slope, m, of a line passing through two points, (x_{1}, y_{1}) and (x_{2}, y_{2}) use this formula: slope = Δy / Δx slope = (y_{2}  y_{1}) / (x_{2}  x_{1}) To calculate the line's yaxis intercept, b, use the formula, y = mx + b and substitute in the values of the slope, m, along with the coordinates of one data point  either (x_{1}, y_{1}) or (x_{2}, y_{2}). You can then solve for b, as it will be the only variable remaining in the equation. The equation of the line can then be written as y = mx + b once you substitute in your exact values for the line's slope and yaxis intercept. Once you know the line's equation you can determine the coordinates of any of its other points.
 Interpolation means to use the equation of the line to determine the coordinate of a point that would fall inside the domain or range of the graph.
 Extrapolation means to use the equation of the line to determine the coordinate of a point that would fall outside the domain or range displayed on the graph.
