find the slope if any:

x+8y=16
x+0=16
x=16

0 +8y=16
8y=16
8y/8=16/8
y=2

this is the part that loses me:
0-2=-2
16-0=16 -2/16=-1/8 is this correct? If not can you help me?

Put the equations into slope-intercept form:

y = mx + b

m = slope
b = y intercept

For your first problem...
subtract x from both sides
x + 8y = 16
8y = -x + 16
divide both sides by 8
y = -(1/8)x + 2
So, the slope is -(1/8)
I am not sure why you set y to 0 in your solution. That will give you the x intercept, but not the slope.