$a) \text{Vì đường thẳng đi qua M(1; 3), N(3; 2)}$
$\Rightarrow \left\{{{a+b=3}\atop{3a+b=2}}\right.$
$\Leftrightarrow \left\{{{a=\frac{-1}{2}}\atop{b=\frac{7}{2}}}\right.$
$b) \text{Vì đường thẳng đi qua P(1; 2), Q(2; 0)}$
$\Rightarrow \left\{{{a+b=2}\atop{2a+b=0}}\right.$
$\Leftrightarrow \left\{{{a=-2}\atop{b=4}}\right.$
$c) \text{Vì đường thẳng đi qua A(2; -9), Q(-1; 12)}$
$\Rightarrow \left\{{{2a+b=-9}\atop{-a+b=12}}\right.$
$\Leftrightarrow \left\{{{a=-7}\atop{b=5}}\right.$
$d) \text{Vì đường thẳng đi qua A(2; 7), Q(-1; 5)}$
$\Rightarrow \left\{{{2a+b=7}\atop{-a+b=5}}\right.$
$\Leftrightarrow \left\{{{a=\frac{2}{3}}\atop{b=\frac{17}{3}}}\right.$
