Row Level Trigger for insert/update to a table
In MySQL you need to make deferent trigger for updating and Inserting;
DELIMITER $$

CREATE TRIGGER before_ITEM _insert
BEFORE INSERT
ON ITEM FOR EACH ROW
BEGIN
   SET NEW.Total = NEW.Quantity * NEW.Price;   
END $$

DELIMITER ;

CREATE TRIGGER before_ITEM _UPDATE
BEFORE UPDATE
ON ITEM FOR EACH ROW
BEGIN
   SET NEW.Total = NEW.Quantity * NEW.Price;   
END $$

DELIMITER ;

But for that purpose you can use generated Columns like
CREATE TABLE ITEM (
ItemNo        int not null,
category      varchar(20) not null,
Quantity      int not null,
Price       decimal(10,2) not null,
Total       decimal(10,2) GENERATED ALWAYS AS (Quantity * Price),
CONSTRAINT pk_item primary key (ItemNo)
);