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a in (-oo:+oo)
(15-10*a)/5 = -3 // + 3
(15-10*a)/5+3 = 0
(15-10*a)/5+(3*5)/5 = 0
3*5-10*a+15 = 0
30-10*a = 0
(30-10*a)/5 = 0
(30-10*a)/5 = 0 // * 5
30-10*a = 0
30-10*a = 0 // - 30
-10*a = -30 // : -10
a = -30/(-10)
a = 3
a = 3
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