So what you are looking for is the hypotenuse of the triangle DCF.
We know DC is 3cm, so we need CF.
BC and CF and CF create a right triangle.
Using Pythagorean Theorem, CF is about 10.2
Now using Pythagorean Theorem again, 3^2 +10.2^2=113
The sqaureroot of that is 10.6. So 10.6 is your answer
