Answer: x = 3
Those angle markings tell us that all three angles are the same measure (60 degrees). So this is an equilateral triangle with each side congruent to each other.
Pick 2 sides, equate the expressions, solve for x. I'm going to pick AB and AC to work with
AB = AC
6x - 3 = 3x+6
6x-3x = 6+3
3x = 9
x = 3
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This works with any pair of sides, such as AC and BC
AC = BC
3x+6 = 5x
6 = 5x-3x
6 = 2x
2x = 6
x = 3
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and let's do the last one to be complete (it's optional)
AB = BC
6x-3 = 5x
6x-5x = 3
x = 3
Either way, we get the same result each time.
