



 
Hello San. The amplitude in the question is 50cm, assuming we interpret the question to mean there is 100cm difference between one extreme and the other (amplitude is measured from the center to either extreme). In the equation, the variable a is the amplitude. If you let y be the variable representing the distance from the center line, then x could be considered the time component of the motion. It is left to determine the value of k, which clearly has some bearing on the period. The sine function alone has a period of 2π, and in this question that takes 10 seconds. So
Just pick x=10 and solve for k. Hope this helps,  


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