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I'm getting started with signal processing, and I'm stuck on a problem that asks me to take the $\mathcal Z$-transform of the following causal, DT-LTI system: $$\sum_{k=0}^M b_k x[n-k] $$

I'm not really sure how to take the $\mathcal Z$-transform with the summation in there. Can anyone point me in the right direction?

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You can use the linearity property of the $\mathcal Z$-transform:

$$a_1x_1[n] + a_2x_2[n] \overset{z}\longleftrightarrow a_1X_1(z) + a_2X_2(z)$$

together with the time shifting property:

$$x[n-k] \overset{z}\longleftrightarrow z^{-k}X(z)$$

Expand the summation like below and apply the two properties.

$$\sum_{k=0}^M b_k x[n-k]=b_0x[n]+b_1x[n-1]+\ldots+b_Mx[n-M]$$

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