Which two linear equations represent perpendicular lines? select two choices?

In order for two linear lines to be perpendicular, the product of their gradient must be - 1.

Let's take y = x + b and y = - x + b

This is already in the form: y = mx + b, where m is 1 and - 1 respectively.

Since the product of their gradient is - 1, they are said to be perpendicular.