You have to differentiate y twice with respect to x
Step 1: differentiate the function with respect of x
[tex]\frac{dy}{dx}=20x^3-12x[/tex]
Step 2: differentiate the function with respect to x again
[tex]\frac{d\square^2y}{dx^2}=60x^{2}-12[/tex]
So d²y/dx²=60x²-12
