B: (0, a)
C should have the same y value as B
D: (a,0)
C should have the same x value as D
So point C is (a,a)
Since point A is on the origin, its point is (0,0)
You use the slope formula and plug in point A and C:
[tex]m=\frac{y_{2}-y_{1}}{x_{2}-x_{1}}[/tex]
[tex]m=\frac{a-0}{a-0}[/tex]
[tex]m=\frac{a}{a}[/tex]
m = 1
So the value that belongs in the green box is 1
