We find the length of AD using the similarity of right triangle property. See image attached.
AD² = BD × CD
AD² = 9 × 36
AD² = 324
AD² = 18²
AD = 18
The length of AD is 18 units
AD² = BD × CD
AD² = 9 × 36
AD² = 324
AD² = 18²
AD = 18
The length of AD is 18 units