Generating function for sequence $a_n = 5\times 7^n - 3\times 4^n$

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How would you proceed to find the generating function of the following sequence? $$a_n = 5\times 7^n - 3\times 4^n$$ I have no idea where to start for cases where the index appears as exponent.

Thanks in advance!

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Hint: The sequence satisfies the recurrence $a_{n+2}=11a_{n+1}-28a_n$.

This comes from expanding $(x-7)(x-4)$.