Question 7 - A-Level Maths

Using the definitions of hyperbolic functions in terms of exponentials, prove that $$\cosh x \cosh y - \sinh x \sinh y = \cosh ( x - y )$$
Given that $\cosh x \cosh y = 9$ and $\sinh x \sinh y = 8$, show that $x = y$.
Hence find the values of $x$ and $y$ which satisfy the equations given in part (ii), giving the answers in logarithmic form.