(a)We have C_t/A = sqrt((e*epsnl)/2)*sqrt(Na/Vj)
sqrt((e*epsln)/2) = 2.91e-016
Hence C_t = 2.91e-016 * sqrt(Na/Vj) F/cm^2
Or C_t = 2.91e-004 * sqrt(Na/Vj) pF/cm^2
(b)The depletion layer capacitance = 68.84 pF 