basic adder problem

Design a circuit that adds together 4X and 8X to give 12X, where X is X2X1X0, and gives a 7-*‐bit sum S6S5S4S3S2S1S0. This can be done using 1 full adder and 2 half adders, no additional gates are required.

all I know is adding 2 bits with one half adder and use bunch of full adder to get a n bit sum...

I have no idea what to do on this..

please help

thanks