Let a = {dirk, johan, frans, sarie}, b = {frans, sarie, tina, klaas, henrika}, c = {hans, frans}. find the number indicated.
n(a ∪ (b ∪ c))

Thank you for posting your question here at I assume the question should be "Find the number indicated". below is the answers:

B U C means B union C which combines them: 
{Frans, Sarie, Tina, Klaas, Henrika, Hans} note that there is no need to write Frans twice. 

∩ means intersection which is only what the two sets have in common, in this case: 
{Frans, Sarie} 

the number is 2.