a) $x^2 - 4x + 3 = 0$

$\Leftrightarrow x^2 - 3x - x + 3 = 0$

$\Leftrightarrow (x - 3)(x - 1) = 0$

$\Leftrightarrow \left[ \begin{matrix}x = 3\\ x = 1\end{matrix}\right.$

b) $2x^2 + 5x + 2 = 0$

$\Leftrightarrow 2x^2 + 4x + x + 2 = 0$

$\Leftrightarrow (x + 2)(2x + 1) = 0$

$\Leftrightarrow \left[ \begin{matrix}x = -2\\ x = \frac{-1}{2}\end{matrix}\right.$

c) $4x^2 - 12x + 9 = 0$

$\Leftrightarrow (2x - 3)^2 = 0$

$\Leftrightarrow 2x - 3 = 0$

$\Leftrightarrow x = \frac{3}{2}$