Let's conside the first equation x+y=8
Since it is an equation with degree 1, it is a straight line. So 2 coordinates are enough to plot a graph.
To plot a grpah, let's take, x=0,1
If x=0

So, the coordinate is (0,8)
If x=1

So, the coordinated is (1,7)
Let's plot the graph.
Let's take the second equation
6x+3y=36
If x=0

Hence the y coordinated is (0,9)
If x=1

Hence the coordinate is (1,10)
Let's plot the graph
Hence both the graphs are plotted.