Is integer x divisible by 21? (1) x is divisible by 14.

(1): x can be 14, which is indivisible by 21, or by 14*21, in which case it is divisible by 21.
Insufficient
(2): x can be 12, which is indivisible by 21, or by 12*21, in which case it is divisible by 21.
Insufficient
Combined: x is divisible by 14 and by 12
x must be divisible by the prime factors of 14=2*7 and by those of 12=2*2*3, thus x is divisible by 7 and by 3
-> x is divisible by 7*3=21.
Sufficient