Determine A and B so that the curve
y = A cos2x + B sin3x
has a point of inflection at ($\displaystyle \Pi/6, 5$).
You know that the point given has to be on the curve in order to be an inflection point. So plug in the values for x and y and you'll get a relation in terms of A and B.
Now find y'' like you normally would to find inflection points. Set y''=0 and now you have a second relation. Two variables, two equations.