Answer:
x=12
Explanation:
14/21=16/(x+12)
cross multiply
14(x+12)=16(21)
14x+168=336
14x=336-168
14x=168
x=12 (so the side would be x+12=24)
You could also see that 14 is 2/3 of 21, so 16 has to be 2/3 of x+12. That means x+12 would have to equal 24. If you take the 12 away from 24, x=12.