Answer :

JcAlmighty
The answers are:
b.-(81)^1/2
c.9 
d.81^1/2

√a = -a^1/2      ⇒ √81 = -81^1/2
√a = a^1/2       ⇒ √81 = 81^1/2

√81 = √9²
√a² = a            ⇒ √9² = 9
√a² = -a           ⇒ √9² = -9