ok so i need to find exact value of Cos75, Sin15 and Cot-210

for the first, i have the identity cos(a-b)=Cos(b)*Cos(a) + Sin(b)*Sin(a)

and im using 120 = a and 45 = b...cause i figured out cos and sin of those using the 30-60-90 and 45-45-90 triangle rules......but i have like: 1/root2 * -.5 + 1/root2 * root3 /2

and i dont know how to do this...i keep getting the wrong answer

so please help and help do the other two

thanks