Đáp án:
$\dfrac{2}{x-3}$
Giải thích các bước giải:
$\dfrac{3x +21}{x^2 - 9} + \dfrac{10}{5 + 15x} + \dfrac{3}{3 - x}$
$=\dfrac{3x +21}{(x-3)(x+3)} +\dfrac{2(x-3)}{(x-3)(x+3)}-\dfrac{3(x+3)}{(x-3)(x+3)}$
$=\dfrac{3x +21 + 2x - 6 - 3x - 9}{(x-3)(x+3)}$
$=\dfrac{2x + 6}{(x-3)(x+3)}$
$=\dfrac{2}{x-3}$