I need help taking the derivative of y = arcsec(2x^2)

This is what I do

y = arcsec(2x^2)

x=sec(2x^2)

y'=-4ysin(2y^2)

I am suppose to get a polynomial but I cannot take the derivative of it to get one. I just get stuck at the last step. Any help? This seems not to make any sense at all for me