Plz can anyone help me?

What number should be added to both sides of the equation to complete this square? x^2-10x=7

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.

What makes a trapezoid an isosceles trapezoid?