Simplify the expression:

cos(θ + π/6) - sin (θ + π/6)

Write your answer as an exact value.

I'm taking grade 12 math through an independent learning course and I'm really stuck on this question, I have no teacher to ask for help, could someone give me some pointers? I tried expanding the brackets but could only simplify to:

√3/2 cos θ - 1/2 sin θ

Which doesn't seem right to me...