I can't use trigger to update info from two tables, MYSQL

Question

Hello i have this problem: I have 2 tables :

Create table Student(
    Num int Auto_increment,
    Name varchar(100),
    Age int,
    Gpa int default 2,
    
    primary key(Num)
);

Create table Grades(
    Num int,
    Grade_BG int default 2,
    Grade_Math int default 2,
    Grade_SUBD int default 2,
    
    Primary key (Num)
);

And i want to make a trigger to update the GPA with the three grades from the other table but i'm having hard time doing it.