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a in (-oo:+oo)
(-1/3)*a-(4/7) = 37/21 // - 37/21
(-1/3)*a-(4/7)-(37/21) = 0
(-1/3)*a-4/7-37/21 = 0
-1/3*a-7/3 = 0 // + 7/3
-1/3*a = 7/3 // : -1/3
a = 7/3/(-1/3)
a = -7
a = -7
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