Then why all the question marks? That was my point. If you do all the steps clearly, there is no need for guessing or for question marks.
1/R = 1/A + 1/B - 1/C
LCD on the Right is ABC. Get all three fractions to that denominator.
1/R = BC/ABC + AC/ABC - AB/ABC = (BC + AC - AB)/ABC
Unless something is zero, then a/b = c/d implies b/a = d/c (If non-zero stuff is equal, so are the reciprocals)
R/1 = R = ABC/(BC + AC - AB)
Show EACH step, clearly, so that you will be able to read it tomorrow and tell what it was you did.