Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$-20^oC$
Substitute the given value of the $F$ into the expression: $C=\dfrac {5}{9}\left( F-32\right) =\dfrac {5}{9}\left( -4-32\right) =\dfrac {5\times \left( -36\right) }{9}=5\times \left( -4\right) =-20$