First, factor out 5:
5 (x² -x -20)
We need two numbers that add to -1 and multiply to -20. -5 and 4 satisfy this.
5(x-5)(x+4)
To solve this, you distribute the -y to both terms, 2 and 4y.
It will look something like this: (2*-y)+(4y*-y)
When you simplify, look for any like terms that can be combined, and if none, then leave it as it is.
Your final answer will be: -2y-4y^2
Change the function into vertex form:
f(x)=x²-10x+16
make a square by (adding half of -10)², which is 25, then subtract it:
f(x)=x²-10x+25-25+16
f(x)=(x-5)²-9
so the vertex is at (5,-9)
another way to do it:
f(x)=x²-10x+16
the vertex is when x=-

, b=-10, a=1 in this case.
x=-(-10/2*1)=5
plug x=5 into the quadratic function, you get f(x)=-9