Negative Variance - help!

I'm currently writing up a scientific project and I've come to a complete standstill when calculating the variance. The formula I am using to calculate Variance (from the textbook) is one that is obtained from Linear regression theory as the variance of the slope of the regression. (apparently anyway!)

I have to do the calculations for 3 different data sets. First two - no problem. Third one.. I am getting a negative value. I've gone over and over and over double checking that I haven't mis-calculated anything but I definitely haven't. What's going on?!! I am SO stuck!