X = {5, 6, 7} ==> X \ Y = {6}
(where X \ Y denotes the complement of X with respect to Y, i.e. elements belonging to X and not belonging to Y. Y contains both 5 and 7, so we remove these from X.)
Y = {5, 7, 9, 10} ==> Y \ X = {9, 10}
Then the symmetric difference is the union of these relative complements:
X ∆ Y = (X \ Y) U (Y \ X) = {6} U {9, 10} = {6, 9, 10}