To find if something is a parallelogram, they 1 set of line segments need to have the the slope. In this equation, Slope equals x1 + x2 divided by y1 + y2 (Or the other way around, x2 + x1 divided by y2 + y1)

For your other point, it could be (-4,5)
If this is confusing, please tell me! I'll try to better explain. It also helps to draw out a graph so you can plot the points.
Answer:
The missing number is -5, which you get by subtracting -3 from -8 ((-8)-(-3)), or adding 3 to -8.
Answer:
(2,3)
Step-by-step explanation:
I did this by using the equation that was given. y=x+1.
This mean that the starting point would be the coordinate (0,1) and the x is the slope, which means that from coordinate (0,1) I will go up one and one to the right until I intersected with coordinate (2,3).
The answer is a because you add the same variables and 5x stays the same