Answer:
s = 3
Step-by-step explanation:
Since the bigger triangle is isosceles, the two smaller triangles are equal to each other because line BD is an angle bisector of <B. We also know that since they are equal to each other, side AD and DC are congruent
knowing this, we can set the equation to 3s = s + 6
subtract s from both sides: 2s = 6
divide both sides by 2: s = 3
