Comparing 3 columns and outputting to 4th column if 2 values match

I have an excel spread sheet with 3 columns; [a,b,c] with thousands of rows that line up in the a,b column. What I need is to search column b for the value in column c, and if it matches, take the value from column a and put it NEXT too column C.

The alternative would be to have column c get sorted into where it matches column b. with all the blank spaces at the bottom of the list. I only care about the values of column a when b and c match. Column c also has blank spaces and will only match with ~30% of column b.

Please help :(