Write down all the subsets of the following set: $\emptyset $

If $aN = \{ ax : x \in N \}$,then the set $3N \cap 7N$ is .....$N$.

State which of the following sets are finite or infinite: $\{ x:x \in N \text{ and } 2x - 1 = 0 \}$

Make correct statements by filling in the symbols $\subset$ or $\not\subset$ in the blank spaces:
${ x:x \text{ is a triangle in a plane} } \dots { x:x \text{ is a rectangle in the plane} }$

What universal set $(U)$ would you propose for the set of right triangles?

Write the following set in the set-builder form: $\{ 3, 6, 9, 12 \}$