They want you to use the subtraction formula
Cos (A-B) =Cos(A) Cos(B) + Sin(A)Sin(B)
=Cos(pi/2)Cos(x/2) + sin(pi/2)sin(x/2)
= 0 * Cos(x/2) + 1 *Sin(x/2)
since we don't know what x is, we have to leave the answer in this form.