How can I turn the equation 2x-y>6 into ax+b format so I can graph it

Add y to both sides and subtract 6 from both sides.

2x - y > 6 ==> y < 2x - 6.

Now just plot the line y = 2x - 6 with a dashed curve and shade the appropriate region.

2x-y>6
This is not an equation. This is an inequality. Graphically, it represents a region to one side of the straight line 2x-y=6.
So, what you want is the equation 2x-y=6 which has to be written as y = 2x - 6.

Add y to both sides:
2x > 6 + y
subtract 6 from both sides
2x - 6 > y
graph y = 2x - 6
the domain under the line of the graph satisfies y < 2x - 6

Add y to both sides and the subtract 6 from both sides
2x - y > 6
2x > y + 6
2x - 6 > y
reversing we get 4 > 3 means 3 < 4
y < 2x - 6