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.
Step-by-step explanation:
Let the number be x. Then x + 10 = 20.
This is all you need to write out. But, for practice, find the solution as well.
The function is defined over these ranges of x:
(-∞, -1) and [-1, 7), that is for x < 7.
Note that
The function is undefined at x = --1 over the left portion of the straight line, but the function s defined at x = -1 for the right portion f the straight line.
The function is undefined at x = 7 and for larger values of x.
Answer: x < 7