$\displaystyle \begin{array}{{>{\displaystyle}l}} a.\ AB\ đi\ qua\ A( 2;4) \ nhận\ \overrightarrow{AB}( 4;-2) \ vtcp\\ AB:\frac{x-2}{4} =\frac{y-4}{-2}\\ hay\ x-2+2y-8=0\\ hay\ x+2y-10=0\\ b.\ Gọi\ M( a;0)\\ d( M;\Delta ) =\frac{|2a+3|}{\sqrt{2^{2} +1}} =\sqrt{5}\\ \Leftrightarrow |2a+3|=5\\ \Leftrightarrow \ 2a+3=5\ or\ 2a+3=-5\\ \Leftrightarrow a=1\ or\ a=-4 \end{array}$