A average Mark overall for all of one student subjects

Lets say i have a student table , a enlistment table that shows SubjectId and Results for each student how would i get the overall average mark for a student that has more than 3 subjects

Create table  a_Student( StudentId int(5) Not null, StudentForename varchar (30) Not null, StudentSurname varchar (45) Not null,  PRIMARY KEY(StudentId) ); Create table a_Enlistment( StudentId int(5) Not null, SubjectId char(6) Not null, results char (2), primary key (SubjectId,StudentId), foreign key (SubjectId) references a_Subject(SubjectId), foreign key (StudentId) references a_Student(StudentId) );   insert into a_Student (StudentId,StudentForename,StudentSurname) values ("33447","Alan","Burns"), ("33672","Mark","Harrison",);     Insert into a_Enlistment (StudentId,SubjectId,Results) values           ("33447","TMP243","78"),          ("33447","AWD169","45"),          ("33447","IIA887","48"),         ("33672","TMP243","57"),        ("33672","AWD169","67"),       ("33672","IIA887","48"), 

So i want lets say Alans overall mark for his 3 subjects how would i do that? Any help is welcomed thanks a lot