Write code statements for a dataframe df for the following :

(a) delete an existing column from it.

(b) delete rows from 3 to 6 from it.

(c) Check if the dataframe has any missing values.

(d) fill all missing values with 999 in it.

Write statement(s) to delete a row from a DataFrame.

Write statement(s) to change the value at 5th row, 6th column in a DataFrame df.

Write statement(s) to change the values to 750 at 4th row to 9th row, 7th column in a DataFrame df.