Hello there

I'm new to these forums but I suspect they will be very helpful for years to come!

I'm trying to study ahead and get a feel for first year of Physics, so I decided to go to MIT's OCW and start going through the course.

I came upon one of these worked problems, but the video accompanying it only goes through half the problem and I am a little confused as to how to proceed. It is a very basic question btw, I know. Here is the question:

Attachment 2525
My Attempt:

my attempt.pdf
Any help or a point in the right direction would be greatly appreciated

Thank you.