This is Equilateral Triangle where all the angles are same.
The total angle of a triangle is 180. So angle 1 + angle 2 + angle 3 = 180. (angle 1 = angle 2 = angle 3)
60 + 60 + 60 = 180
all the angles equal to 180.
Now to find value of x:
We know from above information that the angle is equal to 60.
7x - 3 = 60
7x = 60 + 3
7x = 63
x = 63/7
x = 9
Answer: x = 9