It "can't contain y' " because that was a condition set by the problem!
Your first derivative is wrong because you forgot the y' needed for the derivative of with 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
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.