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a in (-oo:+oo)
(1/7)*(28-(3*a)) = 0
(1/7)*(28-3*a) = 0
1/7*(28-3*a) = 0
28-3*a = 0 // - 28
-3*a = -28 // : -3
a = -28/(-3)
a = 28/3
a = 28/3
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