Question 2 - A-Level Maths

Starting from the definitions of cosh and sinh in terms of exponentials, prove that $$\cosh 2 x = 2 \sinh ^ { 2 } x + 1$$
Find the set of values of $k$ for which $\cosh 2 \mathrm { x } = \mathrm { ksinh } \mathrm { x }$ has two distinct real roots.