Let , where: , and . Use the chain rule to find at .
The Chain rule formula I used is the following: In your first partial, you should an x wrt t not y wrt t but you did the correct derivatives in your work.
And, when ,
When ,
. This is one correct?
Looks fine besides the typo; however, I didn't check your derivatives. I am little confused on what you are asking. You want to know what is but you have done everything as long as your math is correct.