a,
PTTS:
$\begin{cases} x=2+3t\\ y=1+4t\end{cases}$
PTCT:
$\dfrac{x-2}{3}=\dfrac{y-1}{4}$
b,
Ta có $\overrightarrow{n}(3;4)$
$\to$ VTCP: $\overrightarrow{u}(4;-3)$
PTTS:
$\begin{cases} x=-2+4t\\ y=3-3t\end{cases}$
PTCT:
$\dfrac{x+2}{4}=\dfrac{y-3}{-3}$
c,
$\overrightarrow{u}=\vec{AB}(3-3; 4+1)=(0;5)$
PTTS:
$\begin{cases} x=3\\ y=-1+5t\end{cases}$
Đường thẳng không có PTCT