To find the length of segment \( SU \), we can use the Secant-Tangent Theorem, which states that if two secant segments \( SQ \) and \( SU \) intersect outside the circle at point \( S \), then:
\[ (SQ) \times (SR) = (SU) \times (ST) \]
From the information provided:
- \( SQ = 21 \)
- \( SR = 8 \)
- \( ST = 4 \)
- \( SU = x \) (unknown length we want to find)
Now we can set up the equation:
\[ 21 \times 8 = x \times 4 \]
Calculating \( 21 \times 8 \):
\[ 168 = 4x \]
Now, divide both sides by 4 to solve for \( x \):
\[ x = \frac{168}{4} = 42 \]
Thus, the length of segment \( SU \) is:
Segment SU is 42 units long.