I agree with Mqbx. In the question you might consider using the other angle and sin and then sorting out tan at the end. That is, if this is a-level.
Also, a rectangle 9 by 7.5 is only 67.5 and so this parallelogram is only a bit off being a rectangle. There is no way that the angle required is what the answer indicates.
Tan theta needs to be negative if it is greater than 90 and less than 180, which it is.