$M\in ∆\begin{cases}x=1+t\\y=2+t\end{cases}$
`=>M(1+a;2+a)`
$\quad A(-1;2);B(3;1)$
`=>\vec{AM}=(1+a+1;2+a-2)=(a+2;a)`
` \vec{BM}=(1+a-3;2+a-1)=(a-2;a+1)`
$\\$
$∆MAB$ cân tại $M$
`=>AM=BM`
`=>AM^2=BM^2`
`=>(a+2)^2+a^2=(a-2)^2+(a+1)^2`
`<=>a^2+4a+4+a^2=a^2-4a+4+a^2+2a+1`
`<=>4a+4=-2a+5`
`<=>6a=1`
`<=>a=1/ 6`
$\\$
$\quad M(1+a;2+a)$
`=>M(1+1/ 6;2+1/ 6)`
`=>M(7/ 6; {13}/6)`