By definition, a function relates each element in the domain to only one element in the range. This means that each vertical line you draw through the x-axis can intersect the function at only one point. ... y = 2x +1. This is the equation of a straight line with slope 2 and y-intercept 1, so it is a function.
A linear function is two distinct but related notions. So, y=2x-1 is also a linear function.
