i don't know how to simplify the exponent to get the answer
how do i get the answer to 1-(1-.081/12)e-420
The way you have typed the problem, I am not sure which part is the exponent. I suggest that you use the caret (^) symbol to indicate the exponent. For example, if you mean e to the -420 power, you would put e^-420. Is that what you are going for?
Perhaps you could clarify your question with a comment.