The sum of the interior angles of a polygon with n sides is equal to (n-2)*180. The sum of the interior angles of a nonagon is 7*180 = 1260. As it is a regular nonagon, each interior angle is 140 degrees. The interior angle of a square is equal to 90 degrees. Using this, the obtuse angle of the triangle is 360 - (140+90) = 130 degrees. As this is an isosceles triangle the other angles are (180 - 130)/2 = 25 degrees
In a right triangle with sides a, b and c where c is the hypotenuse, the length of the sides is related by: a^2 + b^2 = c^2.
As the length of the hypotenuse of the right triangle is 73 and the length of one of the other sides is 48, the length of the third side is `sqrt(73^2 - 48^2) = 55`