Sorry; the calculation you made of 6.5 ABV is incorrect, given the way the equation was written.
If you remember back to fourth grade math class -- there's something called "Order of Precedence" when it comes to doing calculations. One of the rules is that multiplication and division are performed before addition and subtraction, unless the numbers are grouped with parenthesis. The way the response above is written, the "Og - FG x 131.25 = ABV" implies the FG is multplied by 131.25 first.
This is basic math.
That was my point in my reply above -- where I got the obviously wrong answer of 'negative 131.5'. Far too many homebrewers blindly use formulas to prepare or understand their brew without understanding why we do what we do. My worry is that if we don't understand, then wrong information gets spread around.
Yes, we all understand we multiply the difference in gravities. We are somewhat experienced.
But what if we don't understand it, or don't have that experience? Someone reads "The formula is Og - FG x 131.25 = ABV", applies basic math rules, and gets the wrong answer.
The formula, as written, is incorrect.