#### Answer

$F=C=-40^{o}$

#### Work Step by Step

We start the with Celsius-to-Fahrenheit equation:
$F=\frac{9}{5}C+32$
We wish to know when $F$ and $C$ are equal, so we set $F=C$ and solve for $C$:
$C=\displaystyle \frac{9}{5}C+32$
$5C=5(\displaystyle \frac{9}{5}C+32)$
$5C=9C+5*32$
$5C=9C+160$
$-160=4C$
$4C=-160$
$C=-40^{o}$
Thus $F$ and $C$ are equal at $-40^{o}$ (this can be confirmed in the picture).