How can i write a stored procedure which will perform a calculation and store the result in a table? I have two tables:

CREATE TABLE `siteArea` (
  `length` decimal(6,2) NOT NULL,
  `breadth` decimal(6,2) NOT NULL,
  `Area` decimal(6,2) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


CREATE TABLE `roadArea` (
  `Area` decimal(6,2) NOT NULL,
  `roadWidth` decimal(6,2) NOT NULL,
  `roadArea` decimal(6,2) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


Where 

Area = length * breadth;
roadArea = Area * road_width;


The Column Area from siteArea table has to be updated into Area column of the roadArea table.

Any changes to the Area of siteArea table has to be updated to the roadArea table and roadArea has to perform the calculation automatically without input from user.
You'll need to put a trigger on the siteArea table so that whenever it's updated or inserted, it'll call a stored procedure with an argument representing the new site area and that procedure will calculate the needed values and insert them in a table.

Your question is too ambiguous for me to describe further.