Question: How to specify free variables in linear system?

Let say I have a linear system of equations with 28 unknowns (Z1, Z2, ..., Z28).

I know that dimension of space of solution > 0. I want for all unknowns to be expressed only by (let say: Z20, Z21, ..., Z28).

How to specify these free variables? I'm pretty sure it is implemented in Maple.

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