$2 \cdot 15$ Let $x_{1}$ and $x_{2}$ be two numbers such that $x_{1} \neq x_{2}$, and $3 x_{i}^{2}-h x_{i}=b(i=$ $1,2)$. Then $x_{1}+x_{2}$ equals
(A) $-\frac{h}{3}$.
(B) $\frac{h}{3}$.
(C) $\frac{b}{3}$.
(D) $2 b$.
(E) $-\frac{b}{3}$.
(25th American High School Mathematics Examination, 1974)
[Solution] Suppose $x_{1}$ and $x_{2}$ are the two roots of the quadratic equation $3 x^{2}-h x-b=0$. By Vieta's formulas, the sum of the roots is $\frac{h}{3}$.
Therefore, the answer is $(B)$.