ANSWER:
I believe you wish to calculate the sum of squares total (SST) for this regression analysis. The sum of squares total is 1053.15
Explanation:
The sum of squares total is numerically derived by adding the sum of squares regression (regression sum of squares) to the sum of squares error (error sum of squares). The regression sum of squares here is 752.25 and the error sum of squares is 300.9
This gives us a total sum of squares of 1053.15
Sums of squares tell if a linear regression of one variable (or variables) on another is good or not.
The squared differences between the observed dependent variable and its mean is a measure of the total variability of the data set.
So the SST is equal to 752.25 + 300.9 = 1053.15