Here. The equation gives a regression line which uses the mentor's scores to predict the young trainee's scores. So you would plug in the mentor's scores for to obtain the predicted mentors scores. In this case, it would be better to use Pearson correlation, because we cannot assume that there are rankings. Also the linear regression line is linear. Spearmans coefficient is . Here is a helpful page that calculates the spearman coefficient. Also there is always uncertainty in measurements, so we could also add error bars.