Stuck with plotting a graph!

Could you post an example worksheet, or an example of the values you're using? It's sort of hard to get started without having any idea what you're dealing with.

Generally you'd have a row/column of numbers for each axis, and go from there...

You can use the LINEST formula to get the components of the trendline.
Note that you are doing the reverse of what the trendline is. You want a X value for the know Y where the trend line returns a Y for the known X.
So the LINEST has the x and y values reversed which is why the values do not match trendline.

you need to swap the x and y values so that the known value is an X value. Then using the LINEST function to obtain trend formula components you can calcuate the Y value.

thanks for the help so far but ive realised that i need to use goal seek, also ive been told concentration needs to be on the x axis and that the shape should be like what the graph looks like now (attached)

now ive found this site that explains it all (fortunately using similar results as me)
http://zimmer.csufresno.edu/~davidz/...condOrder.html

i pretty much understand it all but when it reads "We are now read to use Goal Seek to solve our equation. Copy row 32 and Paste it into 35 so you can play with it. Look at the graph and make a guess at what the concentration should be for an absorbance of 0.347."

im lost, the author doesnt explain how to get that row 32 in the first place? iv made a start dont know if its right?

Firstly your formula way incorrect in column C.
You were using both the x and y value as well as multiplying by the constant.

You need to guestimate the required Y value by looking at the table of known values. So for a Absorbance of 2.8 the starting concentration value is 1.

Then goal seek will refine this value to 1.381299