Well let's pretend that decimal wasn't there, and the question was just asking √81. You should know that the answer to √81 is 9. So now, let's put the decimal back in its place so the question is √.81 again. You just have to do the same thing to the answer by adding a decimal in front of it. Therefore:
√81 = 9
√.81 = .9
Your answer is 0.9 :)
