Enable cell formulae in Libre Calc ?

Technically that's not a formula, it's an expression. An incomplete one unless you assume that whatever cell you're inputting it into is the implicit other side of the = sign.

Excel is just setting up a shortcut to the =SUM() formula from the looks of it. So worst case scenario just do =sum(a17+c57-b12) If you remember back to like 5th or 6th grade math, you'll remember that subtraction is really just adding a negative value, which is why there's no =subtract() or =difference() formula.

I also assume you're aware that spreadsheets follow operational order of precedence. So multiply and divide before add, unless it's within parentheses, etc. If you're going to be stringing together formulas operating on more than 2-3 values, it's wise to keep that in mind so you don't get the wrong results. Computers are completely literal and will do only what we tell them to do and it's not possible for developers of programs to divine the intentions of every single possible formula someone might enter into a spreadsheet. You should also keep in mind that if precision is important, Excel is well known to take a number of shortcuts that are less precise. For most people it's a non-issue, but statisticians, bankers, even scientists, may need a level of precision Excel isn't able to give them. If you fall into one of those categories, you may want to be rethinking using a spreadsheet period. Not sure if LibreOffice Calc takes the same shortcuts, but it's probably best to assume it does, same with QuattroPro and any others I'm forgetting.