Take O as the centre of the circle and name the chords as AB and AC.
Make OP such that OP ⊥ AB & AP = PB
Similarly, OQ ⊥ AC & AQ = QC
Given that, AB = 18 units [1 mark]
Consider ∆OPA and ∆OQA,
∠PAO = ∠QAO = x° (Given)
AO = AO (Common side)
∠APO = ∠AQO = 90° (Right angles)
∴ ∆OPA ≅ ∆OQA by AAS congruence.
So, AP = AQ
Hence, AB = AC = 18 units [1 mark]