Asap . what is the meaning of 9.1 in this equation

If we plug in d = 0 (to indicate no desserts are ordered), then we'll get

t = 1.4*d + 9.1
t = 1.4*0 + 9.1
t = 9.1

indicating that, on average, the server expects to get $9.10 in tips if no desserts are ordered

This is why the answer is choice D) 

Period is the time it takes for one “cycle” to complete. in the parent sine function, period is 2*pi since sin(0) = 0, sin(pi/2) = 1, sin(pi) = 0, sin(3pi/2) = -1, and sin(2pi) = 0. at that point, the sequence repeats itself. i’m not sure if your graph is similar to the sine function since you didn’t provide a picture, but you can find the period by looking at a graph.