By PEMDAS, you should first subtract 0.8 from 0.9, because parenthesis should be solved first. This gives you 0.1, but we have to change the 0.1 to -0.1, because of the negative sign before it. Next, multiply -0.1 times four. This gives you -0.4, because opposite signs make a negative when you are multiplying (Or dividing, but that isn't relevant to this problem.) Finally, subtract -0.4 from -.55. (Don't forget to change the signs!) This gives you -0.15, your final answer.