Are constellations bound together by gravity?

No. There's no relationship among the stars in a constellation, and they're not bound together by anything. They may be many light years apart, and just happen to lie in roughly the same direction from the Earth so they appear close together as seen by US.

A constellation could be formed from the tree right in front of you, the cat in the grass next to the tree, and the house and two cars five miles away in the same direction as the tree.

They appear to form a pattern or a picture only because the human brain is hard-wired to look for familiar patterns in random stuff. The constellations exist only in our minds.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.