This probably should be posted in the Advanced Statistics sub forum, maybe a mod will move it.
What values for length do you want to find? What data do you already have? What units for weight and length are you using?
For example if you want to know the weight of a fish length 'a' then simply use your equation
sub value 'a' or whatever you like into x.
This will give you the weight. A better way of describing the equation in your report maybe
where W = weight and L = Length