Linear Functions - Exercise 2 - Modeling

A website owner makes money via advertising on her site.  The owner finds that the approximate monthly net income is approximately equal to $25 when 40 people visit the site daily and $225 when 200 people visit the site daily.  Show that the linear function that gives the monthly net income as a function of number of daily visitors is y = 1.25x - 25 where y represents monthly net income and x represents daily number of visitors. Justify each step.

BONUS POINT - What does the "y-intercept" represent in this problem?