I can do the full equation but the problem is trying to find the angle between two vectors,

So if I had the problem

Find angle between these two given vectors:

A= (2,1,-3)

B= (1,-1,1)

A.B= -2

|a| = sqrt14

|b| = sqrt3

so

-2/sqrt42 = -0.309

I know cos(thetre) = A . B / a b

but I'm still not sure how to go about finding the angle

how do I work out the angle?? Iam quite new to vectors so Iam guessing this is quite obvious,

Please can someone point it out for me!!

I have the answer 108 degrees in my notes I just dont know how to get that!!