|2a+b|=根号[(2a+b)^2]
=根号[4a^2+4ab+b^2]
=根号[12+4*|a|*|b|*cos150°+|b|^2]
=根号[12-6|b|+|b|^2]
=2
12-6|b|+|b|^2=4
|b|^2-6|b|+8=0
|b|=2或|b|=4