null - Simple SQL, Setting default value for multiple columns in a table -

so i'm doing homework class , have been stuck on problem days, apparently i'm not @ google need be.

here is: "change storereps table null values can’t entered in first , last name columns."

my code (does not work):

    alter table storerep modify lastname not null, modify firstname not null; 

my code (does work need able change both columns @ same time):

    alter table storerep modify lastname not null; 

alter table storerep modify (lastname not null, firstname not null);