Find the vertex of the parabola. y = -4x2 - 16x - 11

The standard equation is y=ax^2+bx+k and the vertex way is y=a(x-h)(x-h)+x
so the coordinate for x is -b/2a, for your problem a=-4   b=-16 so the coordinate for x is -(-16)/(2*(-4))=-2. Now you have to replace x=-2 in your equation y=-4(-2)^2-16(-2)-11= 5.
The vertex is (-2, 5)

Xis equal to three

These are the multiples of 15 but what you are probably asking or what are the factors of 15