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a in (-oo:+oo)
(2/7)*a+1/4 = 15/28 // - 15/28
(2/7)*a+1/4-(15/28) = 0
(2/7)*a+1/4-15/28 = 0
2/7*a-2/7 = 0 // + 2/7
2/7*a = 2/7 // : 2/7
a = 2/7/2/7
a = 1
a = 1
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