What does “equally inclined to both axes” mean in regards to a line?

It means that the angles between the line and both the ##x## and ##y## axis are equal.

This in turn means they must be ##45^o## in other words the is ##=1## or ##-1##.

In terms of function this means: ##y=x+b## OR ##y=-x+b## (only ##y=-x## is shown below) graph{ -x [-10, 10, -5, 5]}