hello

i need some help with sequences. i have been faced with a question which i haven't come accross before. i have no examples to this type of question aswel so i really need some help

the question is :

The sum of the first six terms of an artithmetic series is 21, and the seventh term is three times the sum of the third and fourth.

Find the first erm and the common difference.

i will appreichate any help

thank you

kind regards