Equation: y-y1=k(x-x1)

Where (x1, y1) is the coordinate of a point on the coordinate system that passes through the straight line, and k is the slope of the line.

Derivation: If the line L1 passes through the point P1 (x1, y1) and has a slope of k, find the equation of L1.

Suppose point P(x, y) is any point on the line other than point P1. The slope of line PP1 should be equal to the slope of line L1. According to the slope formula of a line passing through two points, k=(y-y1)/(x-x1) (and: x≠x1)

So, straight line L1: y-y1=k(x-x1)