Question

{1,3} U {5,6,7,8} U {7,8,9}

Answer 1

{1,3}
{5,6,7,8}
{7,8,9}

União: juntar todos os termos, sem repetir:
R.: {1,3,5,6,7,8,9}

Answer 2

união de tudo {1,3,5,6,7,8,9}