you actually ended up using the same formula without trealizing it. now you are correct so far. now all that remains is to find

. we need logs for this. and you should remember, whenever you see a variable as a power and you need to solve for the variable, chances are you need logs.

let's pick up where you left off.

.....divide both sides by 1000

......take log base 10 of both sides

...since

......group

's on one side

......since

......since

And now just plug that into your calculator

EDIT: Ah, CaptainBlack beat me to it, oh well