#### Answer

$20$ units

#### Work Step by Step

Step 1: Since the triangles are similar, their corresponding sides are in
proportion.
Step 2: This means that $\frac{9}{15}=\frac{12}{x}$.
Step 3: Solving through cross multiplication: $9x=15\times12$.
Step 4: $9x=180$
Step 5: $x=20$ units
Check:
Step 1: Equating the two proportions to see if they reduce to the same fraction: $\frac{9}{15}=\frac{12}{20}$
Step 2: Simplifying both sides: $\frac{3}{5}=\frac{3}{5}$
Step 3: Therefore, the answer is accurate.