I need help taking the derivative of y = arctan(x+1)

This is what I do

y = arctan(x+1)

x=tan(y+1)

cos^2(y+1) = y'

cos^2(arctan(x+1)+1) = y'

cos^2(((1-sec^2x)^0.5)+2))=y'

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?