How do you graph the inequality #a - b >= 6#?

Perhaps some confusion regarding graphing this is which variable goes on which axis. I will let a be the x-axis, and b the y-axis (this is marginally more challenging than the other way round).

First, we want to rearrange to make b the subject.

#a-b>=6#
#a>=6+b#
#a-6>=b#
#b<=a-6#

To plot this on a graph, we first want to draw on the line #b=a-6#

graph{x-y=6 [-14.08, 17.94, -7.5, 8.52]}

And now, we denote the region which satisfies this inequality. Since our inequality is less than, the region below the line satisfies the inequality. If you aren't 100% sure whether you have the right region, take a point and check.
Eg:
If I thought it was above the line, I might check the point #(5,5)#.

#(5)-5(5)>=6#
#0>=6# which is absurd (slightly), so we know the other region is the 'correct' one. You may wish to check the points#(5,-5), (0,0), (15,2)# or #(7,-1)# as some examples, as an exercise.

Now, we denote our region:

graph{x-y>=6 [-22.94, 34.77, -11.97, 16.9]}

This graph has done this by shading. You may alternatively want to draw an R in the region, and leave a key saying the R is the region for which #a-b>=6# You should also label the line as #a-b>6#