In an arithmetic sequence each term is the sum of the previous term and a fixed ratio. For this case we have the 11th term and the 15th term, so there are 4 steps between the two terms, which means that the 11th term plus 4 times the ratio should be equal to the 15th one.

The ratio is -8. We can use the same idea to determine the 28th term, there are 13 steps between the 15th term and the 28th one. So we have:

The answer is -105.