book Maths Puzzle Numerical Ability

I need help on this one!!

x-3y + 4z = 22
-2x + 5y - 3z = -27
7x + 4y + 2z = 26

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book Other Question

Whats a independent and dependent variable How Do I Solve This Problem?

x-2/2 - x+2/4 = 1/2