To apply the SAS (Side-Angle-Side) Congruence Theorem, two sides of one triangle must be equal to two sides of the other triangle, and the angle included between the two sides must also be equal.
To determine the correct value of \( f \) that allows you to prove the triangles are congruent by SAS, you would typically set up an equation based on the lengths of the sides involving \( f \) and compare the angles that are included between those sides.
Without the specific side lengths, angles, and the exact configurations of the triangles given, I can't calculate a specific value for \( f \) or determine if it can be done at all. If you had additional information about the triangles, I would be able to help you derive the correct equations and solve for \( f \).
To conclude:
- If the included angle and the lengths of two sides can be made equal by setting \( f \) to a specific value, then that value would be your answer.
- If no such value allows for equality with respect to the sides and angle, then the answer is "There is no value of \( f \) that will allow us to prove that these triangles are congruent."
Please refer to the specific problem details to see if any of the values (21, 13, 19) can satisfy the condition for congruence.