##x=105^o##

An equiangular triangle is a triangle whose all interior angles are equal.

As sum of the interior angles of a triangle is ##180^o## each interior angle is ##60^o## and

consequently each exterior angle is ##180^o-60^o=120^o##

as such ##x+15^o=120^o##

##x=120^o-15^o=105^o##