Max and min

Given that $-1 \leq x_i \leq 2$ for $i=1, 2,..., n$ such that $\displaystyle \sum_{m=1}^n x_m = 19$ and $\displaystyle \sum_{m=1}^n x_m ^2 = 99 ,$ find the ratio of the maximum and minimum values of $\displaystyle \sum_{m=1}^n x_m ^3$ .