This looks like a duplicate of your other question, this time possibly better explained. The answer is similar, use comparison and logical operations, together with find:
nodes = [10.2 12.3
21.443 81.31
32.23 43.31];
x = 21.443;
y = 43.32;
noderow = find(nodes(:, 2) == x & nodes(:, 3) == y)
