You may need to refer to SEAB MF26 booklet for the list of trigo formulas. This question can be solved by expanding: acos(2x-tan^(-1)b). Then, you compare the coefficients of the 2 sides of the equation (since b is a constant value, you should be able to infer that acos(tan^(-1)b) and asin(tan^(-1)b) are the coefficients) to get the value of b. Then to solve for a, you simply sub in the value of b that you have calculated.