When we need to calculate the resultant value of them.......
Speed: measure of how fast an object travels
Velocity: measure of how fast, and in what direction, an object travels
Distance: how far an object has traveled on some path
Position: where an object is located in some reference system
Displacement: the difference between an objects starting position and it’s ending position
Answer:
Here is the complete question:
https://www.chegg.com/homework-help/questions-and-answers/magnetic-field-372-t-achieved-mit-francis-bitter-national-magnetic-laboratory-find-current-q900632
a) Current for long straight wire 
b) Current at the center of the circular coil 
c) Current near the center of a solenoid 
Explanation:
⇒ Magnetic Field due to long straight wire is given by (B),where

Plugging the values,
Conversion
,and 

⇒Magnetic Field at the center due to circular coil (at center) is given by,
So 
⇒Magnetic field due to the long solenoid,
Then
So the value of current are
,
and
respectively.