Whenever you're interested in computing the (absolute) difference between two values of a certain variable, you have to subtract the final value and the initial value. This is often written as delta:
[tex] \Delta x = x_{\text{final}} - x_{\text{initial}} [/tex]
So, in this case, the final temperature is -30°F. The initial temperature is 115°F. So, the variation is
[tex] \Delta T = T_{\text{final}} - T_{\text{initial}} = -30 - 115 = -145[/tex]
A negative delta indicates that the value decreased, which is our case.
