Answer:
Quadrant IV
Explanation:
In Quadrant I the numbers are both positive.
In Quadrant II the number on the x-axis is negative while the number on the y-axis is positive.
In Quadrant III both numbers are negative.
In Quadrant IV the number on the x-axis in positive while the number on the y-axis is negative.
Since your plot point was (a, -b), the number in the y-axis is negative meaning it's located in Quadrant IV.