please help with progressions :)

this is a problem from a book..

..i just hope im using the right names a variables and such for things.. ehhh.. :confused:

it goes like this:

in a rising arithmetic progression, the 3rd, 7th and 17th terms are the first 3rd terms of a geometric progression. the sum of these terms is 78.

A) find the Difference of the arithmetic progression and the Quotient of the geometric progression

B) calculate the 4th term of the geometric progression,

and show it is a term in the arithmetic progression.

I am completely stuck with this, so even a hint on how to start would be good.

by the way, I didn't notice any other people asking about progressions in the past.. whats up? :confused: :rolleyes:

thanks in advance :)