If M varies jointly as R and P, and M = 168 when R = 8 and P = 7, write the equation of a proportional model relating M to R and P, and use the given data to find the value of the constant of proportionality, k. If M varies jointly as R and P, and M = 168 when R = 8 and P = 7, write the equation of a proportional model relating M to R and P, and use the given data to find the value of the constant of proportionality, k.