Is this answer wrong:
My HW is not accepting it..
I don't see anything wrong with it. Once you do the u-substitution $\displaystyle u=\sqrt{x}$ to obtain $\displaystyle 50\int sin^3(u)du$ it's obviously just a matter of integrating $\displaystyle sin^3(u)du$ which can be done using the reduction formula for powers of sine. The integral calculator used the correct formula. Perhaps you forgot to include the constant of integration?