Answers

2016-07-20T19:20:49+08:00
Use the formula of getting the common difference of an arithmetic sequence, which is : a2 - a1

(3p+9) - (p+7) = (p+3) - (3p+9)
3p + 9 - p - 7 = p + 3 - 3p - 9
2p + 2 = -2p - 6
2p +2p = - 6 - 2
4p = -8
4p/4 = -8/4
p = -2
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