In this case, $a = x$ and $b = \frac{3\pi}{4}$.
So, we have:
$\sin \left(x-\frac{3 \pi}{4}\right) = \sin x \cos \frac{3\pi}{4} - \cos x \sin \frac{3\pi}{4}$
Now, we can find the values of $\cos \frac{3\pi}{4}$ and $\sin \frac{3\pi}{4}$ using the unit circle or
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