$$\int_0^{x^2} \! \sqrt{t^4+x^3} \, \mathop{dt}$$
unless x depends on t treat it as a constant inside the integral
find$$\mathop{I}(a,b,C)=\int_a^{b} \! \sqrt{t^4+C} \, \mathop{dt}$$
(note this is an eliptic integral)
then
$$\int_0^{x^2} \! \sqrt{t^4+x^3} \, \mathop{dt}=\mathop{I}(0,x^2,x^3)$$