F(x)=3x-1/x^3 f"(x)=? f(x)=x*cosx f"(3)=?

F(x)=3x-1/x^3
f'=((3*x^3)-(2x^2)*(3x-1))/x^6=(3x^3-6x^3+2x^2)/x^6=-3x^3+2x^2/x^6
f"=(-3x^3+2x^2)' *(x^6)- (-3x^3+2x^2)*(x^6)'

f=x*cosx
f'=cosx-x*sinx
f"=-sinx-sinx-x*cosx=-2sinx- x*cosx