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D( z )
z-3 = 0
z+1 = 0
z-3 = 0
z-3 = 0
z-3 = 0 // + 3
z = 3
z+1 = 0
z+1 = 0
z+1 = 0 // - 1
z = -1
z in (-oo:-1) U (-1:3) U (3:+oo)
12/(z-3) = 15/(z+1) // - 15/(z+1)
12/(z-3)-(15/(z+1)) = 0
12/(z-3)-15*(z+1)^-1 = 0
12/(z-3)-15/(z+1) = 0
(12*(z+1))/((z-3)*(z+1))+(-15*(z-3))/((z-3)*(z+1)) = 0
12*(z+1)-15*(z-3) = 0
57-3*z = 0
(57-3*z)/((z-3)*(z+1)) = 0
(57-3*z)/((z-3)*(z+1)) = 0 // * (z-3)*(z+1)
57-3*z = 0
57-3*z = 0 // - 57
-3*z = -57 // : -3
z = -57/(-3)
z = 19
z = 19
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