How do I get the Difference between f20 and f21 and use it in a formula. Say
f20's value is $17.27 and f21's value is $17.32 and I want to multiply that
difference to a value in f22 of $10,500.

"FunctionRookie" wrote:
How do I get the Difference between f20 and f21 and
use it in a formula.

Simply "f20 - f21" or "f21 - f20", depending on your intent.

By the way, you could simply write do "f20 + f21" instead
of SUM(f20:f21).

And to answer the question in your subject line, the
"opposite" of SUM(a1,...) is simply -SUM(a1,...) or
a1-SUM(...), depending on your intent.

Say f20's value is $17.27 and f21's value is $17.32
and I want to multiply that difference to a value in
f22 of $10,500.

I think you mean "multiply by". Depending on your intent,
try:

=(f21-f20)*f22

or

=(f20-f21)*f22