"The string on a conical pendulum is 2m in length an has a tension on the string
holding the object has a mass of 100 N. The radius of motion is 0.4 m and the
period of rotation is 2 seconds. What is the mass of the object?"

Does anybody have any resources to help solve this problem?

the period of rotation is 2 secs ...you want the rotational speed (Omega) in radians per sec ........... two pie radians in a circle .... so omega is pie = 3.1416radians/sec

the force pushing the mass out is m omega squared r ....