Note that the graph contains the point

The 3 comes from the multiplication of the y-value in the parent function by.
Not considering the multiplication by 3 we would have

. Also remember that the available functions are
sin(x-π/2) and cos(x-π/2
).
We can check that at π/2, sin(x-π/2) is sin(π/2-π/2)=sin(0)=0,
and, π/2, cos(x-π/2) is cos(π/2-π/2)=cos(0)=1.
Thus, our function is f(x)=3cos(x-π/2).
Check for π rad: f(π)=3cos(π-π/2)=3cos(π/2)=3*0=0.
and so on...
Answer: f(x) = 3 cos(x − pi over 2)