Answer:
r = 3.75
Step-by-step explanation:
Using Pythagoras' identity in the right triangle ABC
AC² = BC² + AB²
(r + 6)² = r² + 9²
r² + 12r + 36 = r² + 81 ( subtract r² from both sides )
12r + 36 = 81 ( subtract 36 from both sides )
12r = 45 ( divide both sides by 12 )
r = 3.75
