e = special constant roughly equal to 2.718 but it goes on forever without a pattern; it's better to let the calculator handle this value rather than type the decimal digits in.
r = annual interest rate in decimal form
t = number of years
In this case, we have
P = 500
r = 0.06
t = 6
Again we won't type "2.718..." for 'e', and we'll just type it into the calculator as shown below. That way the calculator can handle the digits instead.
