That's a Congruent Triangle question.
The first thing that we should to analize is it: What figure is EFGH? The question say for us that [tex]\overline{EF} \cong \overline{GH}[/tex] and [tex]\overline{EH} \cong \overline{GF}[/tex], therefore, EFGH is a parallelogram because its parallel sides are congruents (equals).
Now, we should to proof that [tex]\triangle EFH \cong \triangle GHF[/tex]. The simbol "≅" means congruence, that is, [tex]\triangle EFH \cong \triangle GHF[/tex] means that the triangles EFH and GHF are equals.
Let's proof:
[tex]\overline{HF}[/tex] is a diagonal of the parallelogram EFGH. When we have a diagonal in a parallelogram, the opposite angles are congruents (look at the picture: the blue angles are congruent and the red angles are congruents too). Therefore, [tex]E\hat{F}H \cong F\hat{H}G[/tex] and [tex]E\hat{H}F \cong H\hat{F}G[/tex].
Both triangle has a comum side: the diagonal [tex]\overline{HF}[/tex]. The diagonal [tex]\overline{HF}[/tex] is between two angles that we know that are congruents, Therefore, by the case ASA (Angle, Side, Angle), we proof that [tex]\triangle EFH \cong \triangle GHF[/tex].
