Since this is an equilateral triangle, all 3 angles are equal, and all the sides are equal.
6x - 3 = 3x + 6
6x -3x = 6 +3
3x = 9
x = 3
we can check it by plugging 3 into each side for x: 6(3)-3 = 18-3 = 15
3(3)+6 = 9+6 = 15
5(3) = 15
So yes, the x = 3