Rodger's brother gave rodger his collection of coins when he left for college. at that time, the value of the collection was $720. twelve years later, the collection was worth $1080. what is the rate of change? dong forget your units.

(0,720)(12,1080)
slope = (1080 - 720) / (12 - 0) = 360/12 = 30 <== slope

y = mx + b
slope(m) = 30
(0,720)...x = 0 and y = 720
sub and find b, the y int
720 = 30(0) + b
720 = b <== ur y int

ur equation : y = 30x + 720

ur slope is telling u that for every year, the value of ur coins goes up by 30 bucks

the y int is just telling u the original value 

C. cos 90 = 0 you can see this in the unit circle