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t in (-oo:+oo)
t+8 = 4*t-13 // - 4*t-13
t-(4*t)+8+13 = 0
t-4*t+8+13 = 0
21-3*t = 0 // - 21
-3*t = -21 // : -3
t = -21/(-3)
t = 7
t = 7
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