what i need to found ? which letter ?

Great job, KID! :D 

N_{next year}  = N_{this year}  +   0.2(N_{this year})( 1 - N_{this year} / K)     .....filling in what we know

N_{next year}    = 3000 + 0.2(3000)(1 - 3000/4000)  = 3150

So.....we need to use this result for next year to find the result two years from now.....and we have

3150 + 0.2(3150)(1 - 3150/4000)   =  about 3284  [rounded] two years from now

This makes sense, since the number of plants increases each year  [ until the "carrying capacity"  of 4000 is reached ]

N = 3000+0.2 (3000 (1-3000/4000))

N=3,150 plants expected in the 1st. year.

Substitute this in the formula above for the 2nd year, you get:

N= 3,284 plants expected in 2 years from now.

thanks CPhill and Guest !