For the triangle, it is 90 degrees because if you have the length of each side, apply the Pythagorean theorem to the triangle. If you get a true statement when you simplify, then you do indeed have a right triangle! If you get a false statement, then you can be sure that your triangle is not a right triangle.
180 degrees is bigger because it is an obtuse angle which means an angle whose measure is bigger than 90 degrees but less than 180 degrees. Thus, it is between 90 degrees and 180 degrees.