The second answer is correct. I'm not sure why you are including the 3x^5 in the numerator of the first.

-Dan

Edit: Oh your second answer was correct, but a little matter of typing. This line has problems:

dy/du = 1/1-u^2 . (u) >>> SO; 1/(1-3x^5)^2 . (u)

First dy/du = 1/(1 - u^2) (watch the parenthesis!), so upon subbing in u = 3x^5 you get dy/dx = 1/(1 - (3x^5)^2) * du/dx. Note the form of the denominator. Also note that it is du/dx that shows up here, not u. The full chain rule is