A mythical warrior swings a 5.6-kg mace on the end of a magically massless 86-cm chain in a horizontal circle above his head. The mace makes one full revolution in 1.8s. Find the tension in the magical chain.

In fact, it's asking you what is the centripetal force on the mace.

You have:
mass, m = 5.6 kg
radius, r = 0.86 m
periodic time, T = 1.8 s

You must find omega (\omega), the angular velocity and use the formulae

F = mr\omega ^2

T = \frac{2\pi}{\omega}