To solve the problem, let's assume the common setup where we have three equations:

Let (x + y = 7), (y + z = 8), (z + x = 9).

Step 1: Add all three equations
((x + y) + (y + z) + (z + x) = 7 + 8 + 9)

Step 2: Simplify the left side
(2x + 2y + 2z = 24)

Step 3: Divide both sides by 2
(x + y + z = 12)

Answer: (\boxed{12})

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