Um, this question has already been answer, but I, as well, want to write my own answer.
I believe the answer to your question is -2. You need to find the which number is increasing to get to 2.5, which in this case is .5. Using this knowledge, you are going to add .5 to -2.5, which should get you -2. Once you are done finding the blank, it is always best to double check your work. You can add .5 to -2 to see if with be the fourth term in the sequence.
I am honestly not good at explaining things, but I wanted to try. But hopefully my explanation helps you. <3