You would use the cosine rule for this as you have 2 lines and are given the angle between them
The angle can be figured out easily as the clock has 12 equal components, meaning each component is 360/12 which is 30
As there are 5 components between A and B the angle is 30*5 =150
You can plug all of this into the cosine rule
a^2= b^2+c^2-2bcCos(A)
So a = sqrt( 10^2 +7^2 -(2*10*7) cos150)
a=16.4 to 3 sf
