What would you do for a Klondike bar?

How do you use the additive inverse to evaluate an expression that uses subtraction in a full sentence

In the figure below, ∠dec ≅ ∠dce, ∠b ≅ ∠f, and segment df is congruent to segment bd. point c is the point of intersection between segment ag and segment bd, while point e is the point of intersection between segment ag and segment df. the figure shows a polygon comprised of three triangles, abc, dec, and gfe. prove δabc ≅ δgfe.

If chord ab is congruent to chord cd, then what must be true about ef and eg?