1. Correct. The reflection happens over segment AB and then you translate 6 units to the right and 3 units up. This will move point A to D, B to F, C to E, to have the triangles line up perfectly.
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2. Problem 1 already confirmed the triangles are congruent based on how they line up perfectly. Another way to prove this is to either use the SAS (side angle side) or LL (leg leg) theorem. With SAS, we need a pair of congruent sides and a pair of angles, with the angles between the two sides. The horizontal and vertical sides can be easily measured. The angles between the two sides are 90 degree angles. Because we have right triangles, we can use LL which is really just a special case of SAS.
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3. Segment EF is 5 units. You can use the pythagorean theorem with a = 3 and b = 4 to solve a^2+b^2 = c^2 to get c = 5. Or you can use the fact that BC = 5 is given to us, so EF must also be 5 as well. This is because of the idea that if two triangles are congruent, then their corresponding pieces must be the same as well.
Real world example: imagine having 2 houses that are perfect clones of each other. If this is the case, then surely their front doors would be identical copies as well. In this analogy, a house is a triangle, while the front door is the segment.
