3x^2 + 9x + 6 = 0
3x^2 + 3x + 6x + 6 = 0
3x(x + 1) + 6(x + 1) = 0
(3x + 6)(x + 1) = 0
3x + 6 = 0 and x + 1 = 0
3x = -6 and x = -1
x = -2 and x = -1
graph B ( the lower one )
for a function to be defined there can only be one value of y for each value of x
graph A has more than 1 value of y for given values of x
for example ( - 1, 4 ) and ( - 1, - 4 ) thus is not a function
graph B only has 1 value of y for a given value of x thus is a function
