My guess is that this should read L=30+0.5W. The fixed length 30 is changed by the weight by the factor 0,5W where W is the weight.
To make a table prepare two columns headed W and L.
In the W column start at 0 and under it write 20, then 40, up to 120.
You should end up with 7 values: 0, 20, 40, 60, 80, 100, 120. In the L column, work out what L should be. For W=0 L=30. For W=20 L= 40; W=40, L=50. So the values of L are 30, 40, 50, 60, 70, 80, 90.
Now you can plot a graph where the horizontal W axis goes in steps of 20 and the vertical L axis goes in steps of 10 starting at 30. So the units only need marks on the axes at a multiple of 20 (W) and 10 (L).
The number 30 is the vertical intercept of the straight line graph. The number 0.5 is the slope.