Let's use an example. Sherry works out 1 hour each weekday, so...
c(5) = 300(5) + 750
c(5) = 1500 + 750
c(5) = 2250 calories
I bet you can figure out what the 300 represents.
y = mx + b, where m = the slope and b = the y-intercept. If b changes from 750 to 375, what happens?
Aaaaand I extra bet you can get the last part.