How does the function inside the rectangular function work?
\$\operatorname{rect}(\cdot)\$ is a function that has value \$1\$ if whatever appears inside those parentheses (it's called the argument 
of the rect function and
I used \$\cdot\$ instead of some algebraic variable as a place holder) 
has value between 
\$-\frac 12\$ and \$+\frac 12\$. Otherwise, when the argument is strictly
smaller than
\$-\frac 12\$ (or strictly larger than \$+\frac 12\$), 
\$\operatorname{rect}(\cdot)\$ has value \$0\$. In your instance, you need to ask


  For what values of \$\omega\$ does \$\displaystyle \frac{\omega - 10}{2\pi}\$ equal a number between \$-\frac 12\$ and \$\frac 12\$?


and a little thought will show, I hope, that \$\omega\$ must be in the interval
from \$10-\pi\$ to \$10 + \pi\$.  If you have trouble deriving this, try and
find the value of \$\omega\$ that makes \$\displaystyle \frac{\omega - 10}{2\pi}\$
equal exactly \$-\frac 12\$ and then, lather, rinse and repeat for
exactly \$\frac 12\$.