The residual of a line is the observed Y value (the numbers in the table) minus the actual value (the Y value you get when you plug in the X value into the equation)... This question is asking you to find the residuals of all the points, square them, and add them...
For example, the observed Y value for -4 is -15. When you plug in -4 into the equation, you get this:
2.5x-7
2.5(-4) -7
-10 -7
=-17
The observed Y val. is -15, while the actual (equation) one is -17. -15 -(-17) is 2. 2^2 = 4
Do this process for all of the points, the add up the squared residuals, and you'll be done!
Hope this helped!