Well this is simple. When ever it comes to a question about finding a variable (y), you always have to think about the original equation of the line. In this example since the line is straight, it is then: y=mx+c.
The information from the table will be used, subtraction will be done to find the equations for both tables which are.
Table 1 (line graph): y= 3/2x
Table 2: y= 5/4x.
What to do now is to substitute x=12 in both equations which are
Y=3/2(12)
=18
Y=5/4(12)
=15
Finally since we are finding the difference. 18-15=$3