because the initial value is 500, you do not have to add a variable next to it. this value will always stay the same. when he raises 15 dollars per minute, that gives you 15t, if t stands for a single minute. this way, the function should result in r=500+15t, because 500 cannot change in any way as that is the cost of his appearance, and 15, because it changes for every minute he performs.
the given expression is
we factor 3 to obtain;
we split the middle term to obtain;
we factor further to get;