Can someone me? how do i solve for the exact value?

sin(α + β) = sin(α) cos(β) + cos(α) sin(β)

sin(α – β) = sin(α) cos(β) – cos(α) sin(β)

These are the sine identities that we have to use 

The angle here is 255, we have to use differnet angles so that we know their sine and cosine. 

255=45+210
210=180+30
This is enough to solve the problem, you need to use algebra now and search for the sine/cosine of 30,45,180 degree.

goog may have it for you