This might take a little imagination, but you need to imagine that the cue travels in a straight line with a total y displacement of (4-1)=3 units and a total x displacement of (4+2)=6 units.
Now you can find the slope of this line...dy/dx=3/6=1/2
Now that we know the slope is 1/2 we simply apply it to the cue position...
y=x/2+b, using the cue position (1,1) to solve for b
1=1/2+b, b=1/2 thus our line is:
y=(x+1)/2
Now the cue ball must travel four units along the x-axis until it meets the rail so:
y=(4+1)/2=2.5
So the point that you need to aim at is (5, 2.5)