Find a number between 364 and 384 that is both a multiple of 7 and 3

If this number is a multiple of 7 and 3, it must be divisible by 21

So 

384 mod 21   =  6

So...this number must be 

304 - 6 = 378

And 378 factors as.. 2 x 3 x 3 x 3 x 7 =  2 x 3 x 3 x 21