Add $\displaystyle -\frac{{17}}{{40}}+\frac{{21}}{{32}}$ and write the result in simplified form.

What is a common denominator:

$\displaystyle -{17}$/$\displaystyle {40}=$/$\displaystyle$     $\displaystyle {21}$/$\displaystyle {32}=$/$\displaystyle$

The unreduced result is: /$\displaystyle$