Im not sure about your question here. If you need to simplify, you can just use your calculator and input cos(75)cos(15)-sin(75)sin(15) and press =.
You will be getting zero (0).
There is and identity for `cos(75)cos(15)-sin(75)sin(15)` .
Recall `cos(A+B) = cosAcosB - sinAsinB.`
Simply, A = 75 and B = 15.
So cos(75+15) = cos(90) = 0.
Therefore, the answer is 0.
We know that cosAcosB-sinAsinB=cos(A+B)
=cos90 (If The angle given is in degree)