Bellman Ford algorithm - unknown weight

Hi to everyone!!! I have the following problem:

I have a directed graph and I want to find out whether there are any negative cycles using Bellman Ford algorithm. I know the weight of all of the edges in this graph but one (the weights of the edges derive from an exercise).

1) Can I use Bellman Ford with one edge's unknown weight?

2) If not, how can I later calculate its weight using Bellman Ford?

I don't want a code but I need help with understanding its application.