Answer:
Theorem for alternate exterior angles.
Explanation:
From the figure attached,
Two lines GN and FH are the parallel lines and a transversal JM is intersecting these lines at two distinct points.
Different pair of angles formed are between these parallel lines,
∠6 ≅ ∠10 [Corresponding angles]
∠8 ≅ ∠10 [Interior alternate angles]
∠6 ≅ ∠13 [Exterior alternate angles]
Therefore, theorem for alternate exterior angles justifies that ∠6 ≅ ∠13.