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a in (-oo:+oo)
(3/7)*a+a+1/4 = 407/28 // - 407/28
(3/7)*a+a+1/4-(407/28) = 0
(3/7)*a+a+1/4-407/28 = 0
10/7*a-100/7 = 0 // + 100/7
10/7*a = 100/7 // : 10/7
a = 100/7/10/7
a = 10
a = 10
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