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t in (-oo:+oo)
(1/3)*t-((1/7)*t)-1 = 11/21 // - 11/21
(1/3)*t-((1/7)*t)-(11/21)-1 = 0
(1/3)*t+(-1/7)*t-11/21-1 = 0
4/21*t-32/21 = 0 // + 32/21
4/21*t = 32/21 // : 4/21
t = 32/21/4/21
t = 8
t = 8
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