The line thas has a negative slope is the blue one.
Remember that the slope gives the change rate between y and x, that is, the slope is defined by
[tex]m=\frac{\Delta y}{\Delta x}[/tex]
This means that for every
[tex]\Delta x[/tex]
we move to the right, then we move the ammount
[tex]\Delta y[/tex]
in the y direction. Therefore the sign of the slope tell us if we move down (if the slope is negative) or up (if the slope is postive) while we move to the right.
