In triangle STU the right angle is at T, ST = 16 cm and SU = 20 cm.

Step 1. The hypotenuse. $SU$ faces $T$, so $SU = 20\ \text{cm}$ is the hypotenuse.

Step 2. Pythagoras. $TU^2 = 20^2 - 16^2 = 400 - 256 = 144$, so $TU = 12\ \text{cm}$.

Step 3. Adjacent to $U$. The sides touching $U$ are $TU$ and $SU$. $SU$ is the hypotenuse, so the adjacent side is $TU$.