Answer
$ -0.96x^{-1.6}$
Work Step by Step
$f''(x)=\displaystyle \frac{d}{dx}\left[\frac{df}{dx}\right]=\frac{d^{2}f}{dx^{2}}$
$\displaystyle \frac{dy}{dx}=\frac{d}{dx}[4x^{0.4}-x]= 4(0.4x^{0.4-1})-1=1.6x^{-0.6}-1$
$\displaystyle \frac{d^{2}y}{dx^{2}}=\frac{d}{dx}[ 1.6x^{-0.6}-1]=1.6(-0.6x^{-1.6})$=$ -0.96x^{-1.6}$
