To differentiate use formula.
multiply factor by power then minus one from power (where x is invloved)
dy/dx = 12x^3 - 24x^2
d^2y/dx^2 = 36x^2 - 48x (this is just a further differentiation)
b) to find stationary point, put value of x into gradient equation (dy/dx) and it should equal to zero (hence stationary)
c) put value of x into second derivative. answer = 48 = positive hence the nature of this point is positive