1) D
y=9/x cannot be a direct variation because there is no constant.
2) A and B
A: -10+y=3x-10
y=3x
It is direct variation
B: The constant is -1/8. It is direct variation
C: y=9x-9. Not direct variation, there is another constant
D: There is also another constant.
3) D
The constant is also the slope.
(10-5)/(2-1)=5/1
k=5
4) 5/2
y=kx
20=8k
k=20/8=5/2
5) 21
y=kx
56=8k
k=7
y=7 x 3
y=21
Hope this helps.