Answer:
9cm
Explanation:
Below, I've labelled the unmarked side lengths as a, b, and c. We're looking for a.
All the side lengths together should add to 38cm, as the perimeter is the sum of all the sides.
So we have:
a+b+c+4+6+10=38
a+b+c+20=38
a+b+c=18
Note on the image how b and c added together will equal a, the same way the 4cm and 6cm sides add together to equal the 10cm side.
So, if a+b+c=18, and b+c=a, we can say a+a=18.
If two a's are 18cm, one a is half this:
a = 18 ÷ 2 =9cm