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