I'm self studying this book: http://www.people.vcu.edu/~rhammack/BookOfProof/
My background is cal1,2,3, ODEs. I mainly want to learn how to prove.
Section 1.4 Ex 10.
We know that:$\text{ P({1,2,3}) = {$\emptyset$, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}} }$
$\text{10. \{X $\in$ P(\{1,2,3\}) : |X| $\le$ 1 \}}$
This means every element in P such that its cardinality is less than or equal to 1. Therefore, the answer is: $\text{ {$\emptyset$, \{1\}, \{2\}, \{3\}}}$