Missing length is 10.3cm to 1.d.p
The clock is a circle divided into 12
360/12=30 degrees for each part
1) 7 to 8
2) 8 to 9
3) 9 to 10
4) 10 to 11
5) 11 to 12
5 parts
30 x 5 = 150 degrees
We've got an angle between 2 lengths - cosine rule comes to mind for me.
a = root 6.2^2 + 4.5^2 - 2x6.2 4.5 x cos 150
= 10.34476764
length is 10.3cm to 1.d.p
(the others are to 1.d.p too)
Hope this helps!