Looks like the derivative of an integral,
[tex]\displaystyle\frac{\mathrm d}{\mathrm dx}\int_0^x\sqrt{1+a^3}\,\mathrm da[/tex]
Recall the fundamental theorem of calculus:
[tex]\displaystyle\frac{\mathrm d}{\mathrm dx}\int_c^{f(x)}g(u)\,\mathrm du=g(f(x))f'(x)[/tex]
where [tex]c[/tex] is some constant.
Directly applying the theorem here gives the derivative,
[tex]\boxed{\sqrt{1+x^3}}[/tex]