Welcome to the concept of the maths.
Here, we can see, that Linear equation always have highest power or degree of its variable as '1' ,
thus here, y = 3/x has degree as -1 and y = x^2 +5 has degree 2 ,
hence,
The equations that are NOT linear are
2.) y = 3/x and 5.) y = x^2 + 5
