Which of the following is the formula to find the slope of the best fit line? a. m = y 2 y 1 x 2 x 1 b. m = y 2 − y 1 x 2 − x 1 c. m = x 2 − x 1 y 2 − y 1 d. m = x 1 − y 1 x 2 − y 2