Waveguide parameter finding

for Frequency Fmin=6GHz and Fmax=12GHz with wave guide attenuation =-0.15 , The available material has a conductivity (σ) of 2.9x107 S/m.The dielectric medium is air with zero conductivity 1. Identify the required design parameters and use a suitable method to find the values that best meet the required specifications. You may consider each parameter in increments of 1mm and 1.5 < a <3 Cm and b< 1Cm. 2. How will the design problem (1) change if the material cost of the waveguide (per unit length) also has to be considered in the form of a constraint a+b<3.5

Please help me in finding the ans for this waveguide question