I'm not sure about the first part....but....for the second....as long as each different value of the independent variable - usually denoted as x - produces only one value for the dependent variable - usually denoted as y - we have a function.....this passes what is known as the "vertical line" test
Example:
y = x^2 is a function because each value of x only produces only one y
But.....
x = y^2 is not .......note y = -2 and y = 2 produce the same x.......thus, one value of x - namely, 4 - is associated with two different y values........
