Using a contour integral of $f(z)=x^{4n+3}e^{-z}$ to show $\int_0^\infty x^{4n+3}e^{-x}\cos x\;dx=(-1)^{n+1}(4n+3)!/2^{2n+2}$

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I am pretty close to finding the right-hand side of the desired proof (see below) but not able to finish it.

https://i.stack.imgur.com/sSsgH.png

I have done this far but, it seems too long to continue.

https://i.stack.imgur.com/pcIUZ.png