Answer:
t(4) = 6 ---- a + 3d = 6
t(8) + t(9) = -72
a+7d + a+8d = -72
2a + 15d = -72
double the first equation
2a + 6d = 12
subtract them
9d = -84
d = -84/9 = -28/3
Explanation:
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