The perimeter is 39.8 So, the first step to this is looking for any tick marks on the triangles sides.
These ticks represent equal. We can see there are marks between A-B and A-D.
Those are there to represent that line AB and line AD are equal. Since we are given that line AB is 12.1cm, we can say line AD is also 12.1 cm.
From there we can take a look at line AC. We can see that these two triangles share a side.
Also the two angle marks are like tick marks. They represent equal. So we can also say angle DAC is equal to angle BAC.
Now we have enough information to conclude that these triangles are are the same. (They are equal to each other).
Now that it is clear they are equal, we know that Line DC (7.8cm) is the same length as line BC because they are the same triangles with the same length sides. So line BC is also 7.8.
Now let's add up all the sides. Remember do not include line AB (the 10cm.) Perimeter is only the outside of the shape.
