Derivative of $$$\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}$$$ with respect to $$$x$$$

The derivative of a sum/difference is the sum/difference of derivatives:

$${\color{red}\left(\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right)\right)} = {\color{red}\left(\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right) - \frac{d}{dx} \left(\sin{\left(x \right)} \cos{\left(a \right)}\right)\right)}$$

Apply the constant multiple rule $$$\frac{d}{dx} \left(c f{\left(x \right)}\right) = c \frac{d}{dx} \left(f{\left(x \right)}\right)$$$ with $$$c = \cos{\left(a \right)}$$$ and $$$f{\left(x \right)} = \sin{\left(x \right)}$$$:

$$- {\color{red}\left(\frac{d}{dx} \left(\sin{\left(x \right)} \cos{\left(a \right)}\right)\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right) = - {\color{red}\left(\cos{\left(a \right)} \frac{d}{dx} \left(\sin{\left(x \right)}\right)\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right)$$

The derivative of the sine is $$$\frac{d}{dx} \left(\sin{\left(x \right)}\right) = \cos{\left(x \right)}$$$:

$$- \cos{\left(a \right)} {\color{red}\left(\frac{d}{dx} \left(\sin{\left(x \right)}\right)\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right) = - \cos{\left(a \right)} {\color{red}\left(\cos{\left(x \right)}\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right)$$

Apply the constant multiple rule $$$\frac{d}{dx} \left(c f{\left(x \right)}\right) = c \frac{d}{dx} \left(f{\left(x \right)}\right)$$$ with $$$c = \sin{\left(a \right)}$$$ and $$$f{\left(x \right)} = \cos{\left(x \right)}$$$:

$$- \cos{\left(a \right)} \cos{\left(x \right)} + {\color{red}\left(\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right)\right)} = - \cos{\left(a \right)} \cos{\left(x \right)} + {\color{red}\left(\sin{\left(a \right)} \frac{d}{dx} \left(\cos{\left(x \right)}\right)\right)}$$

The derivative of the cosine is $$$\frac{d}{dx} \left(\cos{\left(x \right)}\right) = - \sin{\left(x \right)}$$$:

$$\sin{\left(a \right)} {\color{red}\left(\frac{d}{dx} \left(\cos{\left(x \right)}\right)\right)} - \cos{\left(a \right)} \cos{\left(x \right)} = \sin{\left(a \right)} {\color{red}\left(- \sin{\left(x \right)}\right)} - \cos{\left(a \right)} \cos{\left(x \right)}$$

Simplify:

$$- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)} = - \cos{\left(a - x \right)}$$

Thus, $$$\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) = - \cos{\left(a - x \right)}$$$.