Question

find missing angle on a isosceles triangle

The two vertex = 60

Answer 1

let the third angle be x degrees.
60+60+x= 180 degrees
or, 120+x =180
or, x=180-120 = 60 degrees.

Answer 2

it is an equalateraal triangle as 2 angles of 60 degree 3rd one will also be of 60 degreee (angle sum property)