One of your friends gives you $10 for a charity walkathon. Another friend gives you an amount per mile. After 5 miles, you have raised $13.50 total. Write an equation that represents the amount y of money you have raised after x miles.

One of your friends gives you $10 for a charity walkathon -- this is your beginning amount.
Another friend gives you an amount per mile -- you will to calculate this value.
After 5 miles, you have raised $13.50 total --
-- Since one friend gave you $10.00, subtract this from the $13.50 to give you the amount by walking the 5 miles 
-- $10.00 - $13.50 = $3.50
-- Since you walked 5 miles, divide $3.50 by 5 to give you $0.70 per mile earned.

So, an equation that represents the amount of money earned is: y = $0.70x+$10.00.
This is in slope-intercept form; the slope is $0.70 and represents the amount of money earned per mile and the intercept is $10.00 and represents the initial amount of money that you were given.