Hi all

the question is about arithmetic series and i get them but i haven't come across a question like this before and i don't get what to do...

it says the first term of an arithmetic series is 3 and the seventh term is twice the third term.

(i) find the common difference ( i know for the common difference you need to have a constant addition or subtraction rule but i don't get how to work it out :S)

(ii) calculate the sm of the first 20 terms of the series (i know the equation but i don't know how to use it :S )

thank you sooo much and sorry if you've seen questions like these a million times xx