This is a real life problem, but I hope it is ok that I posted it here.

I have an inaccurate postal scale.

I compiled a list of weights given by the incorrect scale versus the correct weights that it should have given.

If I plot them on a graph, they are both straight lines. The incorrect scale weights are lower than the correct weights. The margin of error slowly increases as the amount of weight increases.

It seems that I should be able to derive a formula to calculate the correct weight from the incorrect weight using this data, since there is a definite consistent relationship between the two.

In an abstract sense, if I have 2 straight lines plotted on a graph, starting at 0 and gradually growing apart, how do figure out the formula which describes the relationship between them?

I need to figure out this out for tomorrow, for extremely boring and intricate reasons. Any help would be greatly appreciated as my few years of math studies are far in the past.