The slope of the line in the first quadrant is
[tex]m\text{ = }\frac{7-0}{7-0\text{ }}=\text{ 1}[/tex]
The slope of the second line in the first , 2nd and third quadrant is
[tex]m\text{ = }\frac{2\text{ -0}}{-2-0}=\text{ -1}[/tex]
since the slope of one is negative of the other then they are perpendicular
