This seems like homework, so I am providing guidance only.
With regards to developed torque
What do you mean by the developed torque?
What is the difference developed torque and the output torque, i.e. the shaft torque?
Does this shed any light on how you might calculate the developed torque, given your knowledge of the output torque?
With regards to the frequency of the rotor current:
How many times per second does the stator field spin around the rotor? That is, what is the relative angular velocity of the stator field vs. the rotor field?
Consider the rotor at standstill (100% slip) and the rotor at synchronous speed (0% slip).
As I noted in your other question, you may find it useful to read a textbook on electric machines.
Specifically, I recommend Mulukutla S. Sarma's Electric Machines (WM. C. Brown Publishers, 1985) section §7.4, Polyphase Induction Machine Performance.
Look for this book, or other texts on electric machines, in your university library.
In passing, you haven't said how many poles your motor has. You have said it is 1150 RPM, which I assume means 1150 RPM at rated load, and not 1150 RPM at some other load. From this I infer it is a 6-pole motor (synchronous speed 1200 RPM.)