$$\eqalign{
& m = {x^2} + 5x - 2 \cr
& m = {x^2} + 2.x.{5 \over 2} + {{25} \over 4} - {{25} \over 4} - 2 \cr
& m = {\left( {x + {5 \over 2}} \right)^2} - {{33} \over 4} \cr
& {\left( {x + {5 \over 2}} \right)^2} \ge 0\,\,\forall x \Rightarrow {\left( {x + {5 \over 2}} \right)^2} - {{33} \over 4} \ge - {{33} \over 4} \cr
& \Rightarrow m \ge - {{33} \over 4} \cr
& \Rightarrow GTNN\,\,cua\,\,m\,\,la\,\, - {{33} \over 4} \cr
& Dau\,\, = \,\,xay\,\,ra \Leftrightarrow x = - {5 \over 2} \cr} $$
