take P(x1,y1) and P(x2,y2)
m=(y2-y1)/(x2-x1)
y-y1=m (x-x1) is the actual line
Note, you can choose either of the two points to be (x1,y1) and (x2,y2)
Calculus shows that functions have slopes as well which can be evaluated at any given point as long as it is continuous