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12-((4/5)*x) = 3 // - 3
12-((4/5)*x)-3 = 0
(-4/5)*x-3+12 = 0
9-4/5*x = 0 // - 9
-4/5*x = -9 // : -4/5
x = -9/(-4/5)
x = 45/4
x = 45/4
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