$\text{Đáp án + Giải thích các bước giải:}$
`xy-3x=-19`
`=>x(y-3)=-19=1.(-19)=-1.19`
`\text{Lập bảng :}`
$\begin{array}{|c|c|}\hline x&1&-19&-1&19\\\hline y-3&-19&1&19&-1\\\hline\end{array}$
`=>`
$\begin{array}{|c|c|}\hline x&1&-19&-1&19\\\hline y&-16&4&22&2\\\hline\end{array}$
`\text{Vậy}` `(x;y)=(1;-16);(-19;4);(-1;22);(19;2)`
`----------`
`3x+4y-xy=16`
`=>(3x-xy)-(-4y+12)=16-12`
`=>x(3-y)-4(3-y)=4`
`=>(3-y)(x-4)=4=1.4=(-1).(-4)=2.2=(-2).(-2)`
`\text{Lập bảng :}`
$\begin{array}{|c|c|}\hline x-4&1&4&-1&-4&2&-2\\\hline 3-y&4&1&-4&-1&2&-2\\\hline\end{array}$
`=>`
$\begin{array}{|c|c|}\hline x&5&8&3&0&6&2\\\hline y&-1&2&7&4&1&5\\\hline\end{array}$
`\text{Vậy}` `(x;y)=(5;-1);(8;2);(3;7);(0;4);(6;1);(2;5)`