Gunakan formula luas=πj². luas dah tahu, π pun dah tahu. jadi gantikan sahaja
U must find the radius then multiply it with 2 so then you get the diameter of the circle.
Given that the area of the sector is 110cm²,
(55º/360º) × (22/7) × r² = 110
(121/252) × r² = 110
r² = 110 ÷ (121/252)
r² = (2520/11)
r = √(2520/11)

d = 2r
   = 2 × √(2520/11)
   = 30.27cm

★ You are advised to not round off any answer before the final answer as this will affect the final answer's accuracy. Therefore, you should let the answer be in fraction or round it off to 4 decimal places. When you reach the step of finding the final answer then you can round off the final answer to 2 decimal places. Yes this does matter especially in SPM, your marks will be deducted for less accurate final answer. 

Now let's see if the answer of r is rounded off to 4 decimal places
r = 15.1357

d = 2r
   = 2 × 15.1357
   = 30.2714
   = 30.27cm

★ Now you see how it affects the accuracy of the final answer.