Deduce something is decreasing

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I have been able to do part a, using the definitions of a decreasing sequence. For part b,

I know i need to start off with $a_{n}-a_{n+1}$ to obtain

$$\frac{2^n}{n!}-\frac{2^{n+1}}{(n+1)!}$$ I'm unsure where to go on from here please could you help