If the wind speed is 15MPH and the temperature is -5 what is the wind chill?

Assuming you mean -5 °F (since you used mph, that sounds like a reasonable assumption):

the wind chill would be: -26 °F(-32°C)

Note: Windchill Temperature is only defined for temperatures at or below 50°F and wind speeds above 3 mph. Bright sunshine may increase the windchill temperature by 10°F to 18°F.