Ummm.... I presume the formula you gave is for the average benefit? If so then you are looking for x values such that the y values are close to 800. What I'd do is a search. You know that the year, x, is positive, so set the x part of the window to something like 0 to 100. Set the y to 700 to 900. If the function does not show up in this box, use 100 to 200 for x in the next window, etc. When the function shows up in the window you can zoom in on this a bit by estimating the correct range of values for x and y from where the function is in the window.

That or you can graph the function

and have the calculator search for the zeros. That will give you a good idea how to set your graphing window.

By the way, please use parenthesis. The expression you wrote is

not the similar expression I wrote above.

-Dan