How do you find x when 18²=10²+12²-2*10*12cos (x)

You would have to get everything to one side except for the cosine (x)

It would look somewhat like this

cosx = (18^2-100-144) / - 240

Once calculated out. You find where cosx where equal that answer.

So it would be where does cosx = - 1/3. Then you solve for that