This is an equilateral triangle (a triangle with 3 congruent/equal sides and 3 congruent angles). A triangle is 180°. Knowing this, you can conclude that each angle is suppose to be 60° (60 + 60 + 60 = 180)
Since you know each angle = 60°, you can do this:
∠S = 60°
7x + 4 = 60 Subtract 4 on both sides
7x = 56 Divide 7 on both sides
x = 8
