Question

prove that each angle of an equilateral triangle measures 60 degree

Answer 1

If we consider all angles as x, then, 
angle 1 + angle 2 + angle 3 = 180 degrees
x + x + x = 180 degrees
3x = 180 degrees
x = (180/3) degrees
x = 60 degrees

Answer 2

We can also use angle opposite to qual sides are equal so similarly all angles will be equal suppose x
X + x + x=180
X =60