A question on determine the rate of formation (chemistry)

[Kudo Shinichi]

Homework Statement

For the reaction A-->2B+C, the following data are obtained for [A] as a function of time: t=0min, [A]=0.80M; 8min, 0.60M; 24min, 0.35M; 40min, 0.20M.
Calculate the rate of formation of B at t= 28 min.

The Attempt at a Solution

I have found out that this equation is a first order reaction with equation: y=-0.0345145x-0.2270455
then I plug in the time 28 min into x in this equation and I got the ln=-1.1934515
=exp(-1.1934515)=0.3031730544
the rate for first order of reaction is k* (k=-slope), which is equal to 0.01046386638

I am wondering did I do the problem correctly? because I was told that it is a wrong answer.
any comment or help would be great. Thank you very much.

 

[kava]

Sorry for bumping this, but I am also having the same problem as the OP. I found the k value to be 3.4×10^-2 and the at that time (which is 35 minutes in my case) to be .24M . I am not sure what I am doing wrong, any help would be great.