Example 4: Find the curves that are perpendicular to the parabolas in Example 3.

It is said:

My question is about and why it is so.

The equations for the parabolas in Example 3 are:

I read through Example 3 and I understand to the extent that I follow the arithmetic and I see how the equations for the parabolas were derived.

The in I am having some trouble understanding why that is or what is really being said. I don't think I've really been through this topic of a curve being perpendicular to another curve before.

Thank You...