I need to know the number I think it’s nine but yk I need some clarification

The edges MN and ON are congruent, so the triangle is isosceles. In particular, this means angles M and O are congruent. Angle N is right, so it has measure 90º. The interior angles to any triangle sum to 180º, so
2 • 5xº + 90º = 180º
10xº = 90º
x = 9