When you put it on the shelf, you gave it potential energy. When you knock it off the shelf, that potential energy gets converted into kinetic energy.
So if you have the potential energy when it’s on the shelf, there’s your answer.
1/2 x mass x speed squared is the formula she has
That’s the wrong formula anyway, that applies to constant speed of course so that you have something to put in the forumla.
If you want to use the equations of motion, you want
v^2 = u^2 + 2ax
to calculate its final speed, where x is the distance, a is the acceleration which is g, and u is the initial velocity which is 0.
So v = 5.94m/s
Then E = 1/2 mv^2 which is 14.11J.
Calculating it the easy way, E = mgh (as below) = 14.12.
Thank **** for that, I’d have looked pretty silly 🙂