"Abscissas" means x-values, while "ordinates" means y-values.

So the question means "where on the rectangular system do we have x = y for all points (x, y)?"

In other words, we want a line connecting points like (-3, -3) and (0, 0) and (5, 5) and (700, 700).

The line we want cuts the first and third quadrants in half at 45°. We can write this line as y = x.