Answer :
standard form is
ax+by=c
so
3x-9=7y
add 9 to both sides
3x=7y+9
minus 7y both side
3x-7y=9
ax+by=c
so
3x-9=7y
add 9 to both sides
3x=7y+9
minus 7y both side
3x-7y=9