Let λ^* be the largest value of λ for which the function f_λ(x)=4 λ x^3-36 λ x^2+36 x+48 is increasing for all x ∈ R. Then f_λ^*(1)+f_λ^*(-1) is…

Let $\displaystyle \lambda^*$ be the largest value of $\displaystyle \lambda$ for which the function $\displaystyle f_\lambda(x)=4 \lambda x^3-36 \lambda x^2+36 x+48$ is increasing for all $\displaystyle x \in \mathbb{R}$. Then $\displaystyle f_{\lambda^*}(1)+f_{\lambda^*}(-1)$ is equal to :