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a in (-oo:+oo)
(-3/4)*a = 3/10 // - 3/10
(-3/4)*a-(3/10) = 0
(-3/4)*a-3/10 = 0
-3/4*a-3/10 = 0 // + 3/10
-3/4*a = 3/10 // : -3/4
a = 3/10/(-3/4)
a = -2/5
a = -2/5
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