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b in (-oo:+oo)
b/13 = 2.1 // - 2.1
b/13-2.1 = 0
1/13*b-2.1 = 0 // + 2.1
1/13*b = 2.1 // : 1/13
b = 2.1/1/13
b = 27.29999891
b = 27.29999891
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