Problem
Congruence matches the vertices in order, so AB pairs with BE, BC with ED, and CA with DB.
A fourth grader who can name and draw the line segments in a figure can pair each side with its twin just by following the order of the matching letters.
4.G.A.1Draw A DiagramMatching sides are equal, so BE = 10 cm, BC = 5 cm and DB = 13 cm all follow.
Once you see the repeating rule 'matching sides are the same,' filling in the unknowns is just reading the equal partner—no measuring needed.
4.G.A.1Look For A PatternEach unknown side is the same length as the side it corresponds to in the congruent partner.
Congruent means one triangle can be laid exactly onto the other, and moving a shape that way never changes a length.
The order of the letters says which side comes to rest on which, so every length can be read straight off its partner.
Each vertex of one triangle lands on exactly one vertex of the other, so each side has exactly one partner.
The triangles touch at one point only, so the perimeter is all six sides: 28 + 28 = 56 cm.
Finding a perimeter is adding the lengths of all the boundary sides; splitting it into the two triangles makes the addition easy and shows the answer is just double one triangle's perimeter.
3.MD.D.8Identify SubproblemsWhen two shapes are congruent, matching sides are equal twins—copy the lengths, then add up every outside side to get the perimeter.