When you list the elements in a set always put braces {   }
around them.  I noticed you did not do that and I put them
for you, but you should always put braces around a list of
elements. 

A U B'

Substitute {1,2,4,5} for A

{1,2,4,5} U B'

Substitute {1,3,5,7} for B

{1,2,4,5} U {1,3,5,7}'

The prime ' means the complement set which is all the elements
in the universal set besides those which are in B, which are

So we replace {1,3,5,7}' by {2,4,6}

{1,2,4,5} U {2,4,6}

Union means we combine those two sets into one:

{1,2,4,5,6}    <--Answer  (We do not list 2 and 4 but once)

Edwin

    Use the formula


        n(A U B') = n(A) + n(B') - n(A ∩ B').


     In the formula


         n(A) = 4;  n(B') = 3;  n(A ∩ B') = 2.


     Hence,  n(A U B') = 4 + 3 - 2 = 5.      ANSWER