Given sets are
A = {4, 5, 6} and B = {7, 8}
A ∪ B = {4, 5, 6} ∪ {7, 8}
= {, , , , }.
B ∪ A = {7, 8} ∪ {4, 5, 6}
= {, , , , }
∴ A ∪ = B ∪ .