Acknowledge the student's frustration, while reassuring him/her that s/he is not "dumb".
 
 

For Example:

"Yeah, this stuff can really get tough.  But remember when you used to have such a hard time with the 8's times tables? ("Yeah.")  Well, how much is 8 times 7? ("56").  Right.  How did you learn it ("I kept practicing.").  Yep, and that's what we're going to do with this material.  Hey with a team like us, no schoolwork is going to beat us.  Right?  ("Right.")  Now who's dumb? ("Not me.")