PART A) the initial value of the function is 150 at 0 weeks, that means that there are still 150 songs to be downloaded at the beginning.
The rate of change is -30 songs each week, that is because the amount of songs left to be downloaded decrease by 30 each week.
PART B:
lets use 2 points of the graph p1(0, 150), p2(1, 120)
calculate the slope:
m = (y2- y1)/(x2 - x1) = (120 - 150)/(1 - 0)
m = -30
and now use line equation in form point-slope:
y - y1 = m(x - x1)
y - 150 = -30(x - 0)
y = -30x + 150