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a in (-oo:+oo)
1/7-4*a = (-2/7)*a+29/21 // - (-2/7)*a+29/21
1/7-4*a-((-2/7)*a)-(29/21) = 0
(2/7)*a-4*a+1/7-29/21 = 0
-26/7*a-26/21 = 0 // + 26/21
-26/7*a = 26/21 // : -26/7
a = 26/21/(-26/7)
a = -1/3
a = -1/3
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