Hey, I know this is isn't an electronics forum, but this IS the General Help section. Also, this forum is quite populated and I'm sure somebody here has an answer to my question.

I'm making a small gift for my friend and I'm using 4 yellow LEDs, but I'm a newbie with electronics and I don't know how I would figure out the resistance.

Here is my schematic:




Can anybody tell me what the two resistors should be?

My source power is 9V
The forward voltage of the LEDs is 2.1V
The forward current of the LEDs is 20mA

I appreciate your help. Thanks everyone!