Explanation

(b)
$\lim_{x \rightarrow - \infty}\frac{2x - 1}{\sqrt{x^{2} + 2x + 1}} = \lim_{y \rightarrow \infty}\frac{- 2x - \frac{1}{y}}{\sqrt{1 - \frac{2}{y} + \frac{1}{y^{2}}}}$
[put $x = - y\ \therefore\ \ \ x \rightarrow - \infty\ ie,\ y \rightarrow \infty$]
$= - \frac{2}{1} = - 2$