I cannot give you the exact code because its a homework assignment. But this should lead you towards the answer.
So the signal s(t) is the sinusoidal signal which you have been given. You are told to construct another signal f(t) which should be a square wave with the given parameters.
You can use the function "square" (check the documentation) or alternatively you can generate a random stream of binary bits using randi([0,1],x,y) and sample it using your sampling frequency.
Best of Luck!
