Excel

I need to lookup values in two columns and have a third column show the result but I cannot find the correct formula. e.g if column a = yes and column b = yes then = Yes, however if column a is either yes or no and column b is either yes or no then = No.

Could someone help please - I have worked out the yes/yes = yes but cannot work out the yes/no or no/yes.

Thanks heaps in advance