Choose some numbers for pizza (x), like, x=1, 2, 3, 4, 5
Then find the corresponding values of y for every value of x.
when x=1, and y=20; y=12+8(1)=20
when x=2,y=28
when x=3, y=36
when x=4, y=44
Then go to the graph, identify the point where x=1 on the Pizza-axis, and where y=20 on the cost axis, then trace these points till they intersect, the point of intersection is where the line graph passes.
Repeat the above steps for the point where x= 4 on the Pizza-axis, and y=44 on the cost-axis.