Substitute the sets for the letters

({1, 2, 4, 5}' ∪ {1, 3, 5, 7})'

     {1, 2, 4, 5}' means to put all the OTHER elements of U in a set
                   and none of these:  

({3, 6, 7} ∪ {1, 3, 5, 7})'

     The ∪ means to combine both sets into one: 

({1, 3, 5, 6, 7})'

     The ' means to put all the OTHER elements of U in a set and none of 
     these   

{2, 4} contains 2 elements <--answer

Edwin