Answer
~(r → ~q)
Work Step by Step
It is not the case that if the house is cold then the heater is not working.
→ : if then
r: the house is cold.
~q : the heater is not working
If the house is cold then the heater is not working will be represented as (r → ~q)
It is not the case that if the house is cold then the heater is not working. This will be represented as: ~(r → ~q)