need an update statements

please can any one make for me an update statemets for this select statement

I want to set a.quantity =

select (a.quantity) + (b.quantity)
from a,b
where a.a_id = b.a_id;

I have tried this statements below and it does not work :

update a
set a.quantity = (
select (a.quantity) + (b.quantity)
from a,b
where a.a_id = b.a_id);