With 180° < θ < 270°, we know that both cos(θ) < 0 and sin(θ) < 0.
Then from the Pythagorean identity, it follows that
sin²(θ) + cos²(θ) = 1 ==> sin(θ) = -√(1 - cos²(θ))
and so
sin(θ) = -√(1 - (-8/13)²) = -√105/13
Recall the double angle identity for sine:
sin(2θ) = 2 sin(θ) cos(θ) = 2 (-√105/13) (-8/13) = 16√105/169
