Predict the result
Multiply negative nine by negative thirteen.

Set up the model
Determine the sign before multiplying the magnitudes.
Two negative factors give a positive product.

Work the example
Nine times thirteen is one hundred seventeen.
The product is positive one hundred seventeen.

Check the result
Now multiply negative fifteen by positive three.
Take eight seconds to think before the answer appears.
[Eight-second silent pause to work; the answer remains hidden.]
Different signs give negative forty five.

Interpret the result
For nonzero factors, equal signs give a positive product and different signs give a negative one.