To convert °C to K we usually apply this formula:
T(K) = 273.15 + T(°C)
To convert K to °C we have to apply this one:
T(°C) = T(K) - 273.15
a) Convert 373 K to °C:
T(°C) = 373 - 273.15
T(°C) = 99.85 °C
b) Convert 98 K to °C:
T(°C) = 98 - 273.15
T(°C) = -175.15 °C