For question number (9) since 2x + 4y = 0 and y = -8x, simply substitute the second equation into the first.
2x + 4y = 2x + 4(-8x) = 2x - 32x = -30x = 0
so x = 0 and y = 0
For question (10)
6x + 7y = 20 and y = 2x (substitute 2x everywhere you see y in the first equation). So
6x + 7(2x) = 20
6x + 14x = 20
20x = 20
x = 1
(now substitute 1 for x in any of the equations)
y = 2x = 2(1) = 2
So x =1 and y =2
Question (11)
-3(x) - 5y = 6 and y = -3 so
-3(x) - 5(-3) = 6
-3x + 15 = 6
-3x + 15 - 6 = 0
15 - 6 = 3x
9 = 3x
3x = 9
x = 3
Question (12)
6x - 5y = 22 and y = -8
6x - 5(-8) = 22
6x + 40 = 22
6x = 22 - 40
6x = - 18
x = -18/6
x = - 3
Question (13)
3x + 3y = -18 and y = 2x
3x + 3(2x) = -18
3x + 6x = - 18
9x = -18
x = -18/9
x = -2
Question (14)
-5x - 5y = 0 and y = 8x so
-5x -5(8x) = 0
-5x -40x = 0
-45x = 0
x = 0 and y=8(0)= 0
Question (15)
-5x -3y = 14 and y = -3
-5x - 3(-3) = 14
-5x + 9 = 14
-5x + 9 -14 = 0
9 - 14 = 5x
5x = 9 - 14
5x = -5
x = -5/5 = 5
Question (16)
-3x -y = -24 and y = 3x so
-3x -(3x) = -24
-3x -3x = -24
-6x = - 24
x = -24/-6
x = 4 and if x = 4 and y = 3x, then y = 3(4) = 12
check the calculations and make sure they are correct