Verified answer

So B and C are interchangeable, it only matters what side is A. Side A is opposite of angle a. If this is not clear, another way of saying this is that side A is bordered by angle b and angle c. Side B is bordered by angle c and angle a. Side C is bordered by angle b and angle a. If you draw a picture this is easy to understand.

You get to pick which is which, all that matters is that A is the angle opposite side a.

You also missed the brackets out of the formula:

Cos(A) = ( b^2 + c^2 - a^2 ) / (2bc)

Though I usually start off with it looking like this:

a^2 = b^2 + c^2 - 2bcCos(A)