$A=x^2-5x+2$
$=x^2-2x.\dfrac{5}{2}+\dfrac{25}{4}-\dfrac{17}{4}$
$=(x-\dfrac{5}{2})^2-\dfrac{17}{4}\ge \dfrac{-17}{4}$
$\min A=\dfrac{-17}{4}\Leftrightarrow x=\dfrac{5}{2}$
$B=16x^2+3x-1$
$=(4x)^2+2.4x.\dfrac{3}{8}-\dfrac{73}{64}$
$=(4x+\dfrac{3}{8})^2-\dfrac{73}{64}\ge \dfrac{-73}{64}$
$\min B=\dfrac{-73}{64}\Leftrightarrow x=\dfrac{-3}{32}$