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a in (-oo:+oo)
3/5-((1/2)*a) = 21/110 // - 21/110
3/5-((1/2)*a)-(21/110) = 0
(-1/2)*a+3/5-21/110 = 0
9/22-1/2*a = 0 // - 9/22
-1/2*a = -9/22 // : -1/2
a = -9/22/(-1/2)
a = 9/11
a = 9/11
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