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x in (-oo:+oo)
5*x+6 = 2*x+21 // - 2*x+21
5*x-(2*x)-21+6 = 0
5*x-2*x-21+6 = 0
3*x-15 = 0 // + 15
3*x = 15 // : 3
x = 15/3
x = 5
x = 5
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