You use a line of best fit for a set of data to make a prediction about an unknown value. The correlation coefficient for your data set is -1. How confident can you be that your predicted value will be reasonably close to the actual value?
I can be a little confident; it might be close, or it might be way off.
I can be very confident; it will be very close, but it probably won’t be exact.
I can be certain that my predicted value will match the actual value exactly.
I can’t be confident at all; this is about as close to a random guess as you can get.