No calculation is involved.
The domain of the relation {(2, 4), (0, 0), (-7, 10), (10, -7)} is the list of first numbers. Those are {2, 0, -7, 10}. When they are put in numerical order, the list becomes ...
... {-7, 0, 2, 10} . . . . domain of the relation
The range of the relation is the list of second numbers. Those are {4, 0, 10, -7}. When those are put in numerical order, the list becomes ...
... {-7, 0, 4, 10} . . . . range of the relation