SPM Practice 1 (Arithmetic Progression) : Question 12

Question 12 (Tricky question! Make sure you try it!)

The sum of the first 6 terms of an arithmetic progression is 39 and the sum of the next 6 terms is -69. Find
(a) The first term and the common difference.
(b) The sum of all the terms from the 15th term to the 25th term.