Y = m x + b , where m is the slope. The slope is the rise over the run or the change in x ( rooms cleaned ) over the change in y ( cost ). 125 = m * 1 + b 175 = m * 2 + b ----------------------- b = 125 - m 175 = 2 m + 125 - m m = 175 - 125 m = 50 ( the rate of change ), b = 75 The formula is: y = 50 x + 75 It means that the starting cost will be $75 and that for every room cleaned, the cost will rise for $50.