I am struggling like crazy with question b.

Here is what I have come up with:

But if I then test $M_X'(0)$, I don't get the expected value I calculated in part a:

Where have I gone wrong?
I am struggling like crazy with question b.

Here is what I have come up with:

But if I then test $M_X'(0)$, I don't get the expected value I calculated in part a:

Where have I gone wrong?
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