If an isosceles trapezoid has a base angle that measures 42 degrees what are the other three angle measures.

See the attached figure to better understand the problem

we know that
∡A+∡B+∡C+∡D=360°
∡A=42°
If is an isosceles trapezoid
then
∡A=∡D
so
∡D=42°
and
∡B=∡C
then
2*42°+2*∡C=360°-------> ∡C=[360°-2*42°]/2-----> ∡C=138°
∡C=138°
∡B=138°

therefore

the answer is
the other three angle measures are
 42°
138°
138°