I presume you know that but you might not have noticed that and so .
When in situations likes this, I love drawing pictures.
For the second term, we have a triangle with a base of 1, a hypotenuse of 3, so the other side using pythagoras is given, that we are talking distance, we take the positive.
Hence, we can say.
Now, we set the original equation to theta, as there is some angle that is works out to be,
Take the inverse of the inverse.
Recall cos(a-b) = cos(a)cos(b) - sin(a)sin(b)
Multiply through by 3
This is about all I can whittle it down to..
Brutal question for non-calc.