A and B are points on a circle, centre O. BC is a tangent to the circle. AOC is a straight line. Angle ABO = x°. Find the size of angle ACB, in terms of x. Give your answer in its simplest form. Give reasons for each stage of your working.

(This question is from the Edexcel Higher GCSE paper 2018) As BC is a tangent to the circle, we know that angle OBC must be a right angle (90 degrees)We also know that lines OA and OB are both the same length as they are radii of the circleThis means that angle ABO is the same size as angle BAO - they are both xAs angles in a triangle add up to 180 degrees, the equation we need is:ACB = 180 - 90 (from the right angle) - x - x and this can be simplified toACB = 90 - 2x