First, we write out the prime factorization of each denominator. That's
going to give us 3 x 7 and 2 x 3 x 5. To get our LCD, we simply multiply one of each type of prime number together.
So we'll need to hang onto our 3 x 7, but we can eliminate the 3 from the second set of prime factors. Our LCD will be 3 x 7 x 2 x 5 = 210. Not that small of a number, but it is smaller than it would be if we simply took 21 x 30.