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a in (-oo:+oo)
3*(1/5-((3/10)*a)) = 0
3*((-3/10)*a+1/5) = 0
3*(1/5-3/10*a) = 0
1/5-3/10*a = 0 // - 1/5
-3/10*a = -1/5 // : -3/10
a = -1/5/(-3/10)
a = 2/3
a = 2/3
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