Answer: 91.0%
To find this, create a two way table.
On the top row, put the 18-21 group with the 76 responding then 14 not responding.
On the second row, put the 22-29 group with 262 responding then 35 not responding.
We are looking for people in the 18-21 group, those are the numbers 76 and 14. However, we must also include those in the older group that responded, 262.
So our fraction becomes (76 + 14 + 262 ) / 387 or 91.0%