This is the concept of algebra, a function is defined as a relationship whereby each input has one and only one single output. In our expression; y=-3x^2-7x-6 this can be written as: f(x)=-3x^2-7x-6 such that, given x=0,1,2,.... f(0)=-6, f(1)=-16 f(2)=-32 This implies that y is a function of x.
