Tính các giới hạn

a) \(\lim \frac{{{n^2} - 3n + 5}}{{3{n^2} - 1}} = \lim \frac{{1 - \frac{3}{n} + \frac{5}{{{n^2}}}}}{{3 - \frac{1}{{{n^2}}}}} = \frac{1}{3}\)

b)\(\mathop {\lim }\limits_{x \to - \infty } \left( {\sqrt {9{x^2} + x} + 3x} \right) = \mathop {\lim }\limits_{x \to - \infty } \frac{x}{{\sqrt {9{x^2} + x} - 3x}} = \mathop {\lim }\limits_{x \to - \infty } \frac{1}{{ - \sqrt {9 + \frac{1}{x}} - 3}} = \frac{{ - 1}}{6}\)