Well consecutive means in a row, or one number after the other (for ex. 1,2,3,4). And for this equation it's asking for 4 consecutive numbers of which the sum is -26. To get this you just need to go on a calculator and just try some consecutive numbers, you'll get it pretty quickly. The answer is -5,-6,-7,-8. The sum of those numbers is -26.
Hope this helps!
The answer is 9 sorry if this is wrong
D.4 he should apply 4 treatments to each of the groups