It's the square root of the square root of 81
Then, take the opposite 
of... [DOC] [TLE] Principal Square Root -- from Wolfram MathWorld [PAR] The concept of principal square root cannot be extended to real negative 
numbers since the two square roots of a negative number cannot be 
distinguished until... [DOC] [TLE] SparkNotes: Powers, Exponents, and Roots: Square Roots [PAR] 9 , because 92 = 81 .