I can't figure out the SS xy forthis problem.

In a particular manufacturing process, the useful life of a cutting tool is linearly related to the speed at which the tool is operated. The data in the accompanying table were derived from life tests for the two different brands of cutting tools currently used in the production process. For which brand would you feel more confident using the least squares line to predict useful life for a given cutting speed?

Cutting_Speed_(meters_per_minute) Brand_A_Useful_Life_(hours) Brand_B_Useful_Life_(hours) 30.0 4.2 6.0 30.0 3.5 6.5 30.0 5.1 5.0 40.0 5.1 6.0 40.0 4.0 4.5 40.0 2.5 5.0 50.0 4.4 4.5 50.0 2.8 4.0 50.0 1.0 3.7 60.0 4.0 3.8 60.0 3.0 3.0 60.0 1.1 2.3 70.0 1.6 1.5 70.0 0.5 2.0 70.0 3.0 1.0

I know the SSyy =170.38-45.8^2/15=30.5373333

SSxy =-189...but I don't understand how to get this calculation. The more I look at the formula the more I get confused. What figures are used to comeupwith -189?