What is the minimum size of the stimulus required to reach the threshold?

In a class experiment, the strength of a neural stimulus and the resulting muscle contraction are compared. A single motor neuron that synapses with a muscle fibre is suspended. The other end of the muscle fibre is attached to a mass. If an electrical stimulus is sufficient to cause an impulse in the neuron, the muscle will contract and lift the mass. The following data were obtained from the experiment. Analyze the data, and answer the following questions.

Strength of stimulus (mV)
1
2
3
4
Mass lifted by muscle contraction (g)
0
10
?
?

a) Define "threshold potential". What is the minimum size of the stimulus required to reach the threshold potential for this motor neuron?
b) Explain the all-or-none response. Then predict the mass that could be lifted at 3mV of stimuli and at 4mV of stimuli.
I need help with the second part of each q please