Find the intersection and union for l={-1,1,2} i={-1,3,7}

Intersection means elements both have. Both sets have -1 but they have nothing else in common so the intersection is {-1}
Union means combining the sets so {-1, 1, 2, 3, 7} Note that I only wrote 1 twice. No duplicates allowed!

3cm and 6 and 9 cm that a possibility of that question