Ответ: =36°

Объяснение:

BA=BC=> ∡A=∡C=x

AM=AC=> ∡AMC=∡C=x => ∡AMB=180°-∡AMC=180°-x

=> ΔAMB:  ∡MAB+∡B=180°-∡AMB=180°-(180°-x)= x

AM=MB=>∡MAB=∡B= > ∡B=x/2

=> ΔABC :  ∡A+∡B+∡C=x+x+x/2=2.5x=180°

x=180°:2.5=72°

=> ∡B=x/2=72/2=36°