Bethany is told to create an expression from $2 \square 0 \square 1 \square 7$ by putting a + in one box, a in another, and $a x$ in the remaining box. There are 6 ways in which she can do this. She calculates the value of each expression and obtains a maximum value of $M$ and a minimum value of $m$. What is $M-m$ ?
The six expressions that Bethany creates are

$$
\begin{aligned}
& 2+0-1 \times 7=2+0-7=-5 \\
& 2+0 \times 1-7=2+0-7=-5 \\
& 2-0+1 \times 7=2-0+7=9 \\
& 2-0 \times 1+7=2-0+7=9 \\
& 2 \times 0+1-7=0+1-7=-6 \\
& 2 \times 0-1+7=0-1+7=6
\end{aligned}
$$

Of these, the maximum value is $M=9$ and the minimum value is $m=-6$.

Thus, $M-m=9-(-6)=15$.

ANSWER: 15