To find the missing value in the equation \(100 + 70 + x + 21 = 221\), we first combine the known values:
\[ 100 + 70 + 21 = 191 \]
Now, we can set up the equation:
\[ 191 + x = 221 \]
To isolate \(x\), we subtract 191 from both sides:
\[ x = 221 - 191 \]
Calculating that gives:
\[ x = 30 \]
Thus, the missing value is \(30\).