There is an error in the premise set on one line in the proof. That line is missing an assumption number that should be there, based on that line's justification. Which line number is missing an assumption number in its premise set?

$($Q $->$ R$),(-$S $->-$R$)|-($Q $->$ S$)$

$1(1)($Q $->$ R$)$ A

Aim:

$($Q $->$ S$)$

$2(2)(-$S $->-$R$)$ A

$3(3)$ Q A$/$CP $|-$ S

$1,3(4)$ R $1,3$ MP

$1,2,3(5)--$S $2,4$ MT

$1,2(6)$ S $5$ DN

$1,2,3(7)($Q $->$ S$)3-6$ CP