take two points and
the slope of the line connecting these two points is given by:
so use that to find
when you're done, the equation of the line will be given by the point slope form:
just plug in the values and solve for and you will get something in the form which is what you want.
if all the points, in fact, do not lie on the same line, then find the average slope and use it in the formula i gave you. (by that i mean find the slope between each consecutive pair of points and average them).