What formula to use depends on what you have learned!
Khan gave you a list of formulas often taught for solving these problems without (directly) using calculus. (They are derived from calculus.) You put the question under Calculus, which suggests that you may be expected to use calculus directly (or maybe not).
So, please tell us what you have been learning; if you have been given formulas, tell us what they are (in the form given to you). Then we can guide you in selecting which to use where in your problem.
But more important than formulas is the thinking you do. This will involve more than direct application of one formula. So we'll want to see what you have tried in order to see whether you are thinking in the right direction.
Pretty much right. I'm gradually retiring from community college, at the point of teaching about one class a year and tutoring in the learning center.
But I moved here after 20 years of doing the same thing elsewhere online, so it's not retirement that got me here, it's that site closing. There, it was my twin brother who kept giving the same answer I had just given, or was just about to give, or would have given (or wished I had given). Telepathy indeed!
And I try not to spend too much time on here, because it is addictive and I have obsessive tendencies.