given that
x + y = 12 equation 1
y + z = 20 equation 2
z + x = 18 equation 3
on adding equation 1 + 2 + 3 we get
x + y + z = 25 equation 4 now perform operation eq4 - eq1, eq4 - eq2, and eq4 - eq3
we get
x = 5 ,y = 7 and z = 13 then put the value in eq following
x - y + z = 5 - 7 + 13 = 11 Answer
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