[bonsai@mainserver tests]$cat test.txt
line one
line two
line three
line four
line five

[bonsai@mainserver tests]$THELINE="$(sed -n '/four/=' test.txt)"; sed "$((THELINE-2)),$((THELINE-1))d" test.txt 
line one
line four
line five

[bonsai@mainserver tests]$