Intermediate Algebra for College Students (7th Edition)

$x=x_1 + \dfrac{y-y_1}{m}$
Divide $m$ on both sides of the equation to obtain: $\dfrac{y-y_1}{m} = x-x_1$ Add $x_1$ on both sides of the equation to obtain: $x_1 + \dfrac{y-y_1}{m} = x$