What is the equation for mechanical efficiency of a 3 phase induction motor?
Mechanical efficiency is the real output power over the developed output power. Thus, mechanical losses are basically friction losses. Keeping this in mind, you have to calculate the power flow in the rotor. Once you have the power in the rotor you should subtract the power lost due to resistance of the motor's winding. Your mechanical efficiency is then simply:
    (P_rotor-P_resistive_rotor-P_mech_losses)/(P_rotor-P_resistive_rotor). 

Hint for calculating P_rotor. Draw the per phase equivalent circuit of the induction motor (using the transformer model). From the circuit and the given parameters you should be able to calculate P_rotor. If not then you can refer to this page: https://myelectrical.com/notes/entryid/251/induction-motor-equivalent-circuit to gain a better understanding of modelling induction motors.