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a in (-oo:+oo)
(4/7)*a-((2/7)*a)+1 = 13/7 // - 13/7
(4/7)*a-((2/7)*a)-(13/7)+1 = 0
(4/7)*a+(-2/7)*a-13/7+1 = 0
2/7*a-6/7 = 0 // + 6/7
2/7*a = 6/7 // : 2/7
a = 6/7/2/7
a = 3
a = 3
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