Answer:
y=100x+1000; 3,000 CDs after 20 days.
Explanation:
Y=mx + b is your "slope-intercept form" standard equation, where "M" is your slope and "B" is your Y-intercept normally, but in this case it can be seen as your starting sales point.
1,000 copies are sold the first day. That is your Y intercept
100 copies sold there after so slope is seen like "100 copies sold / 1 day"
100/1 = 100x
so we end up with y=100x+1000 where X = the number of days that have passed and Y = number of CDs sold.
y= 100 ( 20) + 1,000
y = 2,000 + 1,000
y=3,000 CDs sold after 20 days.