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**cosine** Struggling a little with this one.

The fourth term in an arithmetic series is 3k, where k is a constant and the sum of the first six terms of the series is 7k+9

a) Show that the first term of the series is 9-8k

b) Find an expression for the common difference of the series in terms of k.

Given that the seventh term of the series is 12, calculate:

c) The value of k

d) The sum of the first 20 terms of the series

Any help is appreciated, I think if someone just points me in the right direction I should be able to work it out.