Answer: 4
Step-by-step explanation: the definition of a rectangle states that opposite sides of the rectangle are congruent. this means that there is 2 sides that measure 6cm. These two sides added together give us 12cm. By working backwards, we can subtract 12cm from the perimeter (20cm), this gives us 8cm. by diving that by 2 we can find the lenght of the missing sides, 4cm each.