How do you convert $ 2 $ inches into centimeters?

Hint: First derive and write the unitary relationship between the unit inches and centimeters. Then use that relationship to form a conversion relation from inches to centimeters. Then modify that relation for one unit of an inch to the centimeter. Then multiply the given inches in the previous step to obtain the required answer.

Complete step by step answer:
In the metrics system, many types of measurements units are used according to their ease of use. Some are used for measurements of wide ranges such as kilometers, miles, and many used for precise measurements such as millimeters, centimeters.
Inch is used to measure quantities like weight, length, capacity. It is represented by the symbol $ ('') $ . Centimeter is \[1/100th\] of a meter that means $ 1cm=0.01m $ . Also $ 1m=39.97'' $ . So $ 1cm=0.3937'' $ .
So, we got a relationship between centimeters and inches. But we are for conversion of inches to centimeters so we will be needing the opposite relation of this.
 $ 0.3937''=1cm $
To obtain the relation of one inch to a centimeter, divide both sides by $ 0.3937 $ so we get,
 $ \Rightarrow 1''=\dfrac{1}{0.3937}cm $
 $ \Rightarrow 1''=2.54cm $
Now we got the relation for one unit of an inch to a centimeter. To find $ 2 $ inches in centimeter, multiply both sides by $ 2 $ we get,
 $ \Rightarrow 1\times 2''=2.54\times 2cm $
 $ \Rightarrow 2''=5.08cm $
So $ 2 $ inches is equal to $ 5.08cm $.

Note:
There are some basic rules for converting one unit to another. If you need to convert a larger unit into a small unit then multiply with the appropriate value. If you need to convert a smaller unit into a large unit then divide with the appropriate value. The value is the number which relates those units with one another.