Whenever you need to convert a temperature expressed in celsius degrees to fahrenheit degrees, you have to use the following formula:
[tex] ^\circ F = 1.8 ^\circ C + 32 [/tex]
So, if we plug the celsius temperature of -10, we get
[tex] ^\circ F = 1.8 \cdot (-10) + 32 = -18+32 = 14 ^\circ F [/tex]