Answer: [tex]12\ cm[/tex]
Step-by-step explanation:
The first step to solve this exercise is to find the Scale factor.
By definition, the Scale factor is the ratio of the length of any side of one of the figures to the lenght of the corresponding side of the other one.
Based on this, you get that, in this case, the Scale factor is:
[tex]Scale\ factor=\frac{20\ cm}{5\ cm}\\\\Scale\ factor=4[/tex]
Therefore, since you need to find the lenght of the side "A", you must multiply the lenght of the corresponding side of the other figure, by the Scale factor.
Then you get that the lenght of the side "A" is:
[tex]Side\ A=(3\ cm)(4)\\\\Side\ A=12\ cm[/tex]
