Kindly COMPLETE Your Question Next Time :)
This sounds as if it came from a question about correlation, where you had a list of fish lengths and weights and were supposed to plot them on a scatter diagram, with weight on one axis and length on the other and find a line of best fit. If you did all that, you would be able to use the line of best fit to interpolate - that is, predict results from within the range of the known ones. For example, if you had made length the variable for your x-axis and weight your y-axis, you could draw a horizontal line from 15kg on the vertical axis to your (straight, presumably sloping) line of best fit, and then a vertical from where it met the line of best fit to the x-axis, and reading of the result.
But without all of that data, and assuming I have guessed correctly what the question was about, there is no way I can answer your actual question. If you want help with your homework, you have to understand enough of it to be able to include the necessary information with your question.
