series = summation of elements of a sequence
Let's start with the sequence for this problem
25, 19, 13, .....
So you see the trend in the sequence? Can you determine and write out the next elements?
As to the number of terms (elements) to make the summation end at -20
25+19+13 is your start; sum of 57
What's the total when you add the next element after 13?
the one after that? and so on and so on until the sum hits -20
Once you hit -20, count up the elements used start with the three given;
the next one will be the fourth ... How many elements/terms did you need to use?