Calculate tension in rope holding balloon to ground and acceleration as balloon begins to rise when released.?

Question

A hot air balloon is tethered to the ground on a windless day. The envelope of the balloon contains 1200m^3 of hot air of density 0.8kg/m^3. The mass of the balloon (not including the hot air) is 400kg. The density of the surrounding air is 1.3kg/m^3.

I am having trouble with questions concerning tension, and I have no idea how to calculate it using F=ma, since acceleration is not mentioned.

Answer 1

The mass displaced is (1.3-0.8 kg/m^3) * 1200 m^3 = 600 kg

The force is F = m * g = 600 * 9.8 = ~6000 N