It "can't contain y' " because that was a condition set by the problem!

so

Your first derivative is wrong because you forgot the y' needed for the derivative ofwith respect to x.

I have no idea in the world why you think the answer should be a number!

From I would differentiate with respect to x again to get

so that

Now, simply because of the requirement that y' not appear in that formula, go back to to get and use that to replace y' in that formula.