This is the concept of application of geometry; the distance that the ladder should be placed will be given by Pythagorean theorem.
thus;
c^2=a^2+b^2
where, c=hypotenuse, a and b are the legs
hence;
15^2+12^2+b^2
b^2=15^2-12^2
b^2=225-144
b^2=81
getting the square root of both sides;
b=sqrt81
b=9
therefore the ladder should be placed 9 feet from the wall for it to reach 12 feet up the building
So your number is z and you want 9 less z-9
if we zero out f(x), namely make y = 0, we can get the roots or x-intercepts for this quadratic equation
now, the equation is in x-terms, meaning is a vertically opening parabola, so the axis of symmetry will be x = something, a vertical line.
well, we have two x-intercepts, one at -4 and another at 2, and the vertex is right half-way between those guys
-4------------(-1)------------2
so the vertex is at x=-1, namely the axis of symmetry is x = -1.
$300
3%=.03 of something
Find .03 of 10000
10000x.03=$300