Use back$-$substitution to solve the system. $($Enter your answers as a comma$-$separated list. If there is no solution, enter NO SOLUTION If the system has an infinite number of solutions, set $x_2=t$ and solve for $x_1$ in terms of $t.)$

$x_1-x_2=4$

$x_2=3$

$(x_1,x_2)=(,)$