我有d maths 唔識 大家可唔可以教吓我

To tackle this type of problem, you must understand the following pricinple:
For y = A sin nx.
A is the peak value ( amplitude) of the function sin x. For A = 1, peak is 1, for A = 3, peak is 3 etc.
n is the number of cycle ( frequency) of the function. For n = 1, there is 1 cycle in 360 degree, for n = 2, there are 2 cycles in 360 degree etc.
Now y = 3 sin ( x/2).
So A = 3 and n = 1/2, that is peak value is 3 and there is one - half cycle in 360 degree.
So the y - coordinate of P is 3, the x - coordinate of P is 180 degree.