DECLARE @t VARCHAR(100)
SET @t = 'This student has not submitted his last few assignments'

SELECT LEFT(LEFT(@t, 50), LEN(LEFT(@t, 50)) - CHARINDEX(' ', REVERSE(LEFT(@t, 50))))

DECLARE @Max  int

SET @Max=??

SELECT
   REVERSE(RIGHT(REVERSE(LEFT(YourColumnHere,@Max)),@Max- CHARINDEX(' ',REVERSE(LEFT(YourColumnHere,@Max)))))
    FROM YourTable
    WHERE X=Y

DECLARE @OriginalData NVARCHAR(MAX)
    ,@ReversedData NVARCHAR(MAX)
    ,@MaxLength INT
    ,@DelimiterPosition INT	;

SELECT @OriginalData = 'This student has not submitted his last few assignments'
    ,@MaxLength = 45;

SET @ReversedData = REVERSE(
    					LEFT(@OriginalData, @MaxLength)
    				);

SET @DelimiterPosition = CHARINDEX(' ', @ReversedData);

PRINT LEFT(@OriginalData, @MaxLength - @DelimiterPosition);

/*
This student has not submitted his last few assignments
1234567890123456789012345678901234567890123456789012345
*/

static string Cut(string s, int length)
{
    if (s.Length <= length)
    {
        return s;
    }

    while (s[length] != ' ')
    {
        length--;
    }

    return s.Substring(0, length).Trim();
}

DECLARE @text nvarchar(max);
DECLARE @end_char int;
SELECT @text  = 'This student has not submitted his last few assignments', @end_char = 50 ;

WHILE @end_char > 0 AND SUBSTRING( @text, @end_char+1, 1 ) <> ' '
	SET @end_char = @end_char - 1

SELECT @text = SUBSTRING( @text, 1, @end_char ) ;

SELECT @text