With #h(x) = (x^(1/3))/(x^2+1)# you use the quotient rule, which states:
#(f/g)'= (g f' - f g')/ g^2 #
Derivative of #f(x)=x^(1/3)#:
#(1/3)x^(-2/3)#
#f'(x) = 1/(3x^(2/3))#
Derivative of #g(x)=(x^2+1)#:
#g'(x)=2x#
Now plug in:
#((x^2+1)*1/(3x^(2/3)) - (x^(1/3)*2x))/(x^2+1)^2#
#((x^2+1)/(3x^(2/3)) - 2x^(4/3))/ ((x^2 +1)^2#
#((x^2+1)/(3x^(2/3)))/(x^2 +1)^2 - (2x^(4/3))/ ((x^2 +1)^2#
#1/(3x^(2/3)(x^2 +1)) - (2x^(4/3))/ ((x^2 +1)^2#