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a in (-oo:+oo)
58/21 = (-1/6)*a+(2/7)*a+3 // - (-1/6)*a+(2/7)*a+3
58/21-((-1/6)*a)-((2/7)*a)-3 = 0
(1/6)*a+(-2/7)*a+58/21-3 = 0
-5/42*a-5/21 = 0 // + 5/21
-5/42*a = 5/21 // : -5/42
a = 5/21/(-5/42)
a = -2
a = -2
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