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The midpoint of (7,9) and (10,2) is located at the average of x- and y-coordinates of both vertices.

$M=(x,\frac{9+2}{2})\\ M=(x,\frac{11}{2})$

This midpoint is the same for the other two vertices of the square. Let's let the y-coordinates of the first unknown vertex be $y_1$ and the second unknown vertex one be $y_2$. Then, we would calculate the average of those vertices as follows:

$\frac{y_1+y_2}{2}=\frac{11}{2}\\ y_1+y_2=11$

Therefore, the sum of the y-coordinates of the other two vertices is 11.