Not sure what you mean but will be explaining about y = 0 and x =0 anyway.
So for x = 0, that means the value of x is 0.
y = -2(0)+8
y=0+8
y=8
So when x = 0, the value of y is 8 (y = 8) or we can write (0,8)
For y = 0, that means the value of y is 0.
0=-2x+8
-8=-2x
-2x=-8
x=4
So when y = 0, x = 4. We can write as (4,0)