The​ "communication revolution" has given us prepaid phone calling cards. These cards are used to make​ long-distance phone calls from any phone. In a recent year the industry posted sales of ​$. Three years later the sales figure had risen to ​$. What is the increase in sales over the​ three-year period? The increase in sales is ​$ nothing.

Which statement explains how you could use coordinate geometry to prove the opposite sides of a quadrilateral are congruent? a. use the slope formula to prove the slopes of the opposite sides are the same. b. use the slope formula to prove the slopes of the opposite sides are opposite reciprocals. c. use the distance formula to prove the lengths of the opposite sides are the same. d. use the distance formula to prove the midpoints of the opposite sides are the same.