Explanation:
1.
equilateral triangle. all sides and also all angles are equal.
the sum of all angles in a triangle is always 180°.
so,
6x = 180/3 = 60°
x = 10
2.
all 3 angles are equal, so, also all 3 sides are equal.
6x - 5 = 5x
x - 5 = 0
x = 5
3.
again, all 3 angles are equal.
3x = 180/3 = 60
x = 20
4.
the 2 sides are marked as equally long.
4x = 40
x = 10
5.
isoceles triangle with 60° angle in top, that means it is again an equilateral triangle.
3x + 8 = 4x - 4
8 = x - 4
12 = x
6.
again all angles are equal.
4x = 60
x = 15