equation or function?

+0.3°c -0.1°c +0.4°c

Looking for either an equation or function for this, in order to simply enter column A & B and have answer autofill.

The number in column A is the constant number that I want column B to be ie: what must happen to column B to equal column A, as you can see in column C you add 0.4 so that column B equals +0.3. As seen below there is variation from positive and negative numbers in both columns therefore at any given time it will be adding or subtracting??

+0.2°c -0.2°c +0.4°c

+0.3°c -0.3°c +0.6°c

0.0°c -0.1°c +0.1°c

-0.1°c +0.3°c -0.4°c

-0.2°c 0.0°c -0.2°c

-0.1°c +0.2°c -0.3°c

anyone help me out, could save some considerable productivity time?