Your answer is going to be fifteen plus r
Answer:
schläfli symbol {50}, t{25}
Coxeter diagram
Symmetry group Dihedral (D50), order 2×50
Internal angle (degrees) 172.8°
5 more rows
Step-by-step explanation:
All of the angles in a triangle add up to 180°.

The missing angle is 146°
You can not work it out because we do not know the shape or the other properties of this poster.
Sorry for not being able to answer it.