I assume you want this to happen whenever something is edited? The .search() method can be called strings to see if a substring is present. If it is, it returns the index of where it was found. If it is not found, it returns -1.
This is an onEdit trigger, and it will get called the moment anything on your sheet is changed. So the moment the text "LET AGREED" is saved in a cell, the row will disappear.
Also, if you are copy and pasting into this sheet a range of data where the words "LET AGREED" will be present in many rows, it will be able to handle deleting all of those rows.
function onEdit(e) {
var ss = e.source;
var sheet = ss.getSheets()[0];
var range = sheet.getDataRange();
var values = range.getValues();
var rows_deleted = 0;
for (var i = 0; i < values.length; i++) {
for (var j = 0; j < values[i].length; j++) {
var value = values[i][j];
//row numbers are 1-based, not zero-based like this for-loop, so we add one AND...
//every time we delete a row, all of the rows move down one, so we will subtract this count
var row = i + 1 - rows_deleted;
//if the type is a number, we don't need to look
if (typeof value === 'string') {
var result = value.search("LET AGREED");
//the .search() method returns the index of the substring, or -1 if it is not found
//we only care if it is found, so test for not -1
if (result !== -1) {
sheet.deleteRow(row);
rows_deleted++;
}
}
}
}
};