I'm not sure on 8 (maybe because I can't see all of option D, but idk)
7.) you get y on one side m and the rest on the other
so it was -3x-7y=21 then you add 7y to both sides to get rid of it (+ gets rid of -, × of ÷, etc)
so now it's -3x=21+7y
then subtract 21 from both sides to get
-3x-21=7y
the y is still not alone; it's being multiplied by 7, so divide it all by 7
(-3x/7)-(21/7)=(7y/7) which makes
y=(-3/7x)-3
9.) slope intercept form is mx+b
the m is the slope, so you're looking for m
but the equation -6x+2y=3 isn't in slope intercept form, you have to convert it
get y on its own side
you can subtract it, making it -6x=3-2y
and then subtract the 3 making it -6x-3 = -2y
then the y isn't alone so you divide everything by -2
making it y=3x+(3/2)
the slope (or m) is 3