This should be easy enough to do, but not sure what you mean by your values in C3, are you trying to subtract $20 from $27 or are you trying to subtract some kind of cell values? If you are always going to subtract $20-$27, would that always just be -$10?
And if that is the case your value, regardless or the value in B3, would always be -$10 as all those calculations would always equal that same amount.