An isosceles triangle is a triangle that has 2 congruent/equal sides and 2 congruent angles.
Side AB and side BC are the same length (the red tick marks shows that the sides are congruent)
Since they are the same length, you can set them equal to each other
Side AB = Side BC
x + 17 = 2x - 6 Subtract x on both sides
17 = x - 6 Add 6 on both sides
23 = x
Now that you know "x", you can find the length of Side BC:
2x - 6 Plug in 23 for "x"
2(23) - 6
46 - 6 = 40
The length of BC is 40
