why is -3 1/8 a rational number

Here is your answer:

The reason why the fraction -3 1/8 is a rational number is because its a fraction, it does not repeat, and also its a negative fraction.

Reason: Irrational numbers are more likely numbers with the pie equation for ex: √144 but it doesn't repeat which makes it a rational number.

Just remember that irrational numbers repeat.

Hope this helps.