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t in (-oo:+oo)
39/21 = t/7 // - t/7
39/21-(t/7) = 0
13/7-1/7*t = 0 // - 13/7
-1/7*t = -13/7 // : -1/7
t = -13/7/(-1/7)
t = 13
t = 13
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