Use the formula for sum of arguments


    cos(a+b) = cos(a)*cos(b) - sin(a)*sin(b)    (1)


You need to know sin(a) and sin(b) and cos(b)


sin(a) = +/-  = +/-  = +/-  = +/- ,

and since  sin(a) < 0, according to the condition, we chose  sin(a) = -.    (2)



Next, b =  = ;

therefore, sin(b) = -;  cos(b) = .    (3)


Now the last YOUR step is to substitute  (2) and (3) into the formula (1).


You do it and complete the calculations on your own.