After ploting all the constrants on the graph in linear programming, from there how does one calculate the iso contribution line, do I have to make an eastimate on the total contribution required or there is another method of coming up with it
To draw the line, you use any total contribution you want because the angle/slope/gradient of the line will be the same whatever the contribution is, and it is only the angle/slope/gradient of the line that is important.
If you watch my lecture on this website then I give tips on what total contribution to use to draw the line.