Since the graph of the given functions is a straight line, the graph belongs to a linear function, as all linear functions have their graphs as a straight line. For a non-linear function the graph is not a straight line.
For a relation (non-function), the line passes through same value of x for more than one points.
-4 plus or minus the square root of 4^2 - 4 * 1 * 4 divided by 2 * 1 = -4 plus or minus the square root of 16 - 16 / 2 = -4 plus or minus the square root of 0 / 2 = -4 / 2 = -2.
