Question

What transformation transforms (a, b) to (a, −b)​?

Answer 1

A translation of 1 unit down

Answer 2

A reflection over the x-axis

If you have -b in your coordinate, it basically means that you're flipping it vertically...

for example if I have (a,b) equals to (2,3)... (a,-b) would be (2,-3) ... which means that rather than 3, it is found -3 ... which is a reflection across the x-axis.