The electric-field phasor of a uniform plane wave traveling

The electric-field phasor of a uniform plane wave traveling downward in water is given by

Where zˆ is the downward direction and z = 0 is the water surface. If ????4 S/m,
(a) Obtain an expression for the average power density,
(b) Determine the attenuation rate, and
(c) Determine the depth at which the power density has been reduced by 40 dB.