C^2 goes in the first box because it’s the only missing variable from the ones given.
Then c^2 goes in the second box because it hasn’t been changed.
Then 2 goes in the third box because (1/2ab x 4 = 4/2 ab = 2ab)
Then c^2 goes in the last box because it hasn’t been changed either, plus it completes the Pythagorean formula (a^2 + b^2 = c^2) so it seems the most logical