slant upward and downward

[hhtt21]

I cannot understand the prepositions upward and downward in "If a line slants upward it has a positive slope and if a line slants downward it has a negative slope."

Another example: "If a line slants upward from left to right, it has a positive slope." Here I can understand "left to right" but "upward" does not make sense to me. Would you explain it?

Source: Algebra and Trigonometry by Keedy/Bittinger.

Thank you.

 

[hhtt21]

'Upward' is in the direction of the top of the page.

Here the context is about the line and mathematics. I can understand a line slants left to right but upward seems to me not necessary.

Thank you.

 

[SoothingDave]

We read from left to right. When you look at a line from left to right, it will either rise or fall. That is upward or downward.

On an x-y graph, as the value of x increases, does the y value increase or decrease? If it increases, the slope is positive and it can be said to slant upward.