can someone please help me with this problem. i'm doing my review right now and i'm stuck?

Question

Find cos(s + t) given that cos s = 1/3, with s in quadrant 1, and sin t = -1/2, with t in quadrant 4.

Answer 1

s in I
if cos s = 1/3, then sin s = √8/3

t in IV
if sin t = -1/2, then cos t = √3/2

cos(s+t)
= cosscost - sinssint
= (1/3)(√3/2) - (√8/3)(-1/2)
=(√3 + √8)/6

Answer 2

this is the wrong answer..

Answer 3

I am 100% sure my answer is right.

What have they got?
Is is (√3 + 2√2)/6 perhaps.

If it is, I will let you think about it.

Answer 4

yes, (√3 + 2√2)/6 is the correct answer/form. thank you for your help!