To find the length of segment \( SU \), we can use the Power of a Point theorem, which states:
\[ (SQ)(SR) = (SU)(ST) \]
From the problem, we know:
- \( SQ = 21 \)
- \( SR = 8 \)
- \( ST = 4 \)
Let's denote the length of segment \( SU \) as \( x \).
Now we can substitute the known values into the Power of a Point equation:
\[ (21)(8) = (x)(4) \]
Calculating the left side:
\[ 21 \times 8 = 168 \]
Now we have:
\[ 168 = 4x \]
To solve for \( x \), divide both sides by 4:
\[ x = \frac{168}{4} = 42 \]
Thus, the length of segment \( SU \) is \( 42 \) units.
Therefore, the correct response is: Segment SU is 42 units long.