I think that the easiest way to solve this problem is to plug in numbers and find the pattern.
First, I started by finding two numbers that make the equation true. I started with x=2, y=1.5. When I plug this into the equation, I get 6+3=9, which works.
Now, I need to increase x by 2, so x=4. Now, I get 12+2y=9, so y= -1.5. y has changed from 1.5 to -1.5, so it has decreased by 3.
Next, I'll plug in a couple more numbers to see if this pattern holds true. If x=1, then y=3. 3+6=9. If I increase x to 3, then y must be 0; 9+0=9; y still decreases by 3.
I'll try one more number to make sure, this time a negative. If x=-1, then y=6. -3+12=9. So if I increase x to 1, y must equal 3. 3+6=9. y still decreases by 3.
The answer is that y decreases by 3
I would use this same method to solve the other questions in this set, but I would only plug in one or two numbers because I know this method works.
For example, if x=1, then y=3. If x decreases by 6, then it would equal -5, and y would have to equal 12. -15+24=9. y has gone from 3 to 12, which means it has increased by 9.
I hope that at this point, can solve the rest of the set on your own!