How to say a negative number in Mandarin?
Negative numbers are simply read 负xxx, for example:

-5 负五
-1234.5 负一千二百三十四点五
(-3) x 5 = -15 负三乘以五等于负十五
2^(-3) 二的负三次方

When applied to certain domain, there might be domain specific way to read it. For example for temperature, it's more common to read it as 零下 (literally 'below zero'):

-10°C 零下十（摄氏）度

Reading as 负十度 is also correct, just less common.
Reference:
人教版小学四年级数学 第七单元 生活中的负数