A negative number multiplied by another negative number gives a positive number.
Rule of Signs
See also
Negative, PositiveExplore with Wolfram|Alpha
References
Derbyshire, J. Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics. New York: Penguin, pp. 367-368, 2004.Referenced on Wolfram|Alpha
Rule of SignsCite this as:
Weisstein, Eric W. "Rule of Signs." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/RuleofSigns.html