This is an equilateral triangle (a triangle with 3 equal/congruent sides and 3 congruent angles)
Since you know the sides have to be the same/equal to each other, you can set the sides equal to each other (you can just do two because all of the sides are suppose to be the same anyways)
BC = AC
5x = 3x + 6 Subtract 3x on both sides
2x = 6 Divide 2 on both sides
x = 3
