Calculus (3rd Edition)

The inflection point is at $t=2$. Concave up on $t\in (2, \infty)$ Concave down on $t\in (-\infty, -2)$.
We have $$y=t^3-6t^2+4, \quad y'=3t^2-12t, \quad y''=6t-12$$ The inflection point is when $y''=6t-12=0$, that is $t=2$. Concave up when $y''\gt 0$, which occurs at $t\in (2, \infty)$ Concave down when $y''\lt 0$, which occurs at $t\in (-\infty, -2)$.