Complete the following calculations.

a) If [[$ 4^2 = 16 $]], then [[$ \sqrt {16} = $]]

b) If [[$ 8^2 = 64 $]], then [[$ \sqrt {64} = $]]

c) If [[$ 11^2 = 121 $]], then [[$ \sqrt {121} = $]]

d) If [[$15^2 = 225 $]], then [[$ \sqrt {225} = $]]