I am attempting to find a particualr line in a file and then replace it. Unfortunatly i can only guaruntee the content of the line prior to the one i am trying to edit. I have used grep -n and a cut to get the line number of the previous line and i now want to be able to return the line after this one. I know part of the string but this returns several lines i have tried:
grep -n $string $file | grep $line_number: $file
where $line_number is the previous line number + 1 and $string is the only consistent part of the line but this returns nothing.

Can someone suggest a solution using grep and if not maybe using awk.

Here are two sed solutions and one awk solution.  Of course, sed and awk both can identify the line prior using pattern match instead of exact match.

sed '/the line prior/{N;s/\(.*\n\).*/\1mynewline/;}' myfile

sed '/the line prior/{x;N;s/.*/mynewline/;x;G;}' myfile

awk '{
print
if ($0=="the line prior")
   {getline
    print "mynewline"}
}' myfile

grep -n "term to find" file.txt | cut -f1 -d:

also works nicely. I use it in Bash scripts to set variables to pass to sed deletes.