Using triangle BCD find the distance between Rex and a dolphin in between Rex and Paulo assume the distance between Palladia dolphin is exactly 100 feet. Pls need help

In order for the function to be linear, what must m be and why? m = 22 because the rate of change is 1. m = 20 because the rate of change is 3. m = 22 because the rate of change is -1. m = 20 because the rate of change is -3.