the temperature is - 4°f. A high pressure front increases the temperature to 8°f. by how many degrees did the temperature increases?

The temperature had to rise 4 degrees to reach zero, and then another 8 degrees to reach 8. The total increase was (4 + 8) = 12 degrees.

A more formal way to do it:

The change is (end value) minus (start value)

Change = (8) - (-4) = (8) + (4) = 12 degrees.

The temperature increased by 12 degrees.