Answer
$\{(-5,-4)\}$
Work Step by Step
1. Given $\begin{cases} x-3y=7 \\ -3x+4y=-1 \end{cases}$ use elimination, multiply 3 to the first equation, we have $\begin{cases} 3x-9y=21 \\ -3x+4y=-1 \end{cases}$
2. Add up the two equations to get $-5y=20$, thus $y=-4$
3. Use the first equation to get $x-3(-4)=7$, thus $x=-5$
4. Thus the solution set is $\{(-5,-4)\}$
