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n in (-oo:+oo)
-3*n-3 = -21/4 // + -21/4
-3*n-(-21/4)-3 = 0
21/4-3*n-3 = 0
9/4-3*n = 0 // - 9/4
-3*n = -9/4 // : -3
n = -9/4/(-3)
n = 3/4
n = 3/4
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