Jack and Jill are hiking on a mountain. Jack's hike is modeled by the linear equation d=0.7t+3 and Jill's hike is modeled by the linear equation d=−0.7t+3, where d is the distance, in miles, from the base of the mountain and t is the time, in hours, since the start of the hike.

a. What is Jack's rate of change?

b. For Extra Credit, explain how Jack is hiking.