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5*x+(21/2)*y = 45 // - 45
5*x+(21/2)*y-45 = 0
5*x+21/2*y-45 = 0 // - 21/2*y-45
5*x = -(21/2*y-45) // : 5
x = (-(21/2*y-45))/5
x = (45-(21/2*y))/5
x = (45-(21/2*y))/5
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