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x +12 -sqrt(x) = 42
-sqrt(x) = -x+30
-sqrt(x)^2 = (-x+30)^2
x^2 - 61x + 900 = 0
(x-36) (x-25) = 0
x = 36
36+12 = 48 - sqrt(36) = 48-6 = 42
X = 36