I am not sure where to start, either, because I am not sure exactly what you need to do. If I had to guess, here's how I would expect to approach this:

1) Extract the "known" data points from the table and put them somewhere convenient. Maybe put 120 into E2, 180 into E3, 2.8 into F2, and 2.25 into F3.
2) Using whatever regression tool you like (LINEST(), LOGEST(), chart trendline, or Data anaylysis->Regression) to perform the regression using those two known points.
2a) In order to do this, you will need to know something about the regression equation you want to use. It sounds like you don't want a simple straight line, but that will mean thinking through the kind of equation you want. Recognize that, with only two data points, your regression equation can have no more than two adjustable parameters.
2b) If you have no idea what kind of equation to use, the best I can recommend is to look at how others in your field model cattle weight gain. If that information is not available to you, review the basic curves you learned in algebra/calculus and choose one: http://www.dummies.com/education/mat...ebraic-curves/
http://tutorial.math.lamar.edu/Class...monGraphs.aspx
3) With parameters for your chosen equation obtained from the regression, enter appropriate formula into column B and copy down (pay attention to relative and absolute references to make copying easy).

What specific step do you need help with?