How would you find the equation for a linear function when you are given a table of values for the variables?

If the figures in the table are exact and without measurement error then take any two of the points (x1, y1) and (x2, y2) and use these to form the linear relation

y - y1 = ((y2 - y1)/(x2 - x1))(x - x1)

If, however, you suspect that the values in the table do not exactly follow a linear relationship then use linear regression for which formulae are provided in wikipedia.