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a in (-oo:+oo)
2/7-4*a = -((1/2)*a)-39/35 // + -((1/2)*a)-39/35
(1/2)*a-4*a+2/7-(-39/35) = 0
(1/2)*a-4*a+2/7+39/35 = 0
7/5-7/2*a = 0 // - 7/5
-7/2*a = -7/5 // : -7/2
a = -7/5/(-7/2)
a = 2/5
a = 2/5
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