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d in (-oo:+oo)
(-2/3)*d+1/3 = (6/7)*d-25/21 // - (6/7)*d-25/21
(-2/3)*d-((6/7)*d)+1/3-(-25/21) = 0
(-2/3)*d+(-6/7)*d+1/3+25/21 = 0
32/21-32/21*d = 0 // - 32/21
-32/21*d = -32/21 // : -32/21
d = -32/21/(-32/21)
d = 1
d = 1
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