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c in (-oo:+oo)
(-10/21)*c = -15/28 // + -15/28
(-10/21)*c-(-15/28) = 0
(-10/21)*c+15/28 = 0
15/28-10/21*c = 0 // - 15/28
-10/21*c = -15/28 // : -10/21
c = -15/28/(-10/21)
c = 9/8
c = 9/8
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