http://tunerspec.ca/school/math.jpg

For the sixth question, I got:

$\displaystyle cos a = \frac{-3}{8}$

$\displaystyle cos b = \frac{-5}{4}$

$\displaystyle sin 2a = \frac{7}{8}$

Whenever I use the answer I got, it says it's incorrect, but every time I do it I get that answer.