Let x represent the number of days after release and y represent the number of books sold.
The first day there were 500 books sold, after that, 150 books were solf each passing day.
This means that for the first day y=500 ann each passing day 150 books were added, the equation is:
y=500+150x
Using this equation you have to calculate the number of books solf after x=50 days.
To do so replace in the equation above:
y=500+150*50
y=8000
After 50 days 8000 books were sold