You use a line of best fit for a set of data to make a prediction about an unknown value of the correlation coefficient for your data set is -0.015. how confident can you be that you're predicted value will be reasonably close to the actual value?

The r value is r = -0.015 which is much closer to 0 than it is to -1, so this means that the linear regression line is a bad model to estimate the points. You're better off using some other model (eg: quadratic model), or it's possible that the data isn't connected at all and it's random noise.

If r were closer to -1, then we'd be more confident that the lines all fall around the same straight line that slopes downward.

what is the original number? you forgot to add it.