Answer:
AC = 18 mm
Step-by-step explanation:
See the attachment for a diagram.
Since length BD = 24 mm, length BO = 12 mm. Then ΔBOC is a right triangle with one leg 12 and hypotenuse 15. The other leg (OC) is given by the Pythagorean theorem:
OC² +OB² = BC²
OC = √(BC² -OB²) = √(225 -144) = √81
OC = 9
Diagonal AC is twice the length of OC, so is ...
AC = 2·9 = 18 . . . . mm
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It can save a little time if you recognize that the given sides of triangle BOC have the ratio 4:5. This suggests you're dealing with a 3:4:5 right triangle, and that side OC is (3/5)·(15 mm) = 9 mm.
