# If we pick two items from a set $M$ where $|M|\geq 2$, how many subsets can be made using the first, second or both items?

Suppose $M$ is a set s.t. $|M|\geq 2$. If we pick two items from $M$, how many subsets can be made using either the first, second or both items?
I wrote $M=\{x_1,x_2,\ldots\}$ and wrote it out for $|M|=2,\dots,5$ and found that $\#$ subsets were 3,6,12 and 24 respectively. From this I got a hunch that the answer could be $3\cdot 2^{|M|-2}$?
You empirically obtained the correct answer. To show it, note that, from the remaining $N-2$ items, we can form $\binom {N-2}{0}$ subsets containing zero elements, $\binom {N-2}{1}$ subsets containing one element, $\binom {N-2}{2}$ subsets containing two elements, and so on. This leads to $$\sum_{k=0}^{N-2} \binom{N-2}{k}=2^{N-2}$$ total subsets. Multiplying this to the $3$ possible initial choices (first item, second item, or both), you get the result.