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

Where (x1, y1) is the coordinate of a point over a straight line in the coordinate system, and k is the straight line slope.

Derivation: If the straight line L1 passes through the point P1 (x1, y1) and the slope is k, find the L1 Equation.

Suppose point P(x, y) is arbitrarily different from point P1 on the straight line, straight line PP1 slope should be equal and straight line L1 slope, According to radical straight line slope formulas, k=(y-y1)/ (x-x1) (and: x≠x1)

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