Given two triangles, we can say that they are congruent by the SAS postulate (Side Angle Side) if both triangles have two congruent sides and the angle that they form is also congruent
In this case, we have that triangle IHG and DFE have already two congruent sides, then, to make them congruent, the angle that they each form (angle IHG and angle DEF) must be congruent so we can use the SAS postulate
