You need to take f(x) and make a perfect square...
x^2+10x+35=0 move the constant to the other side...
x^2+10x=-35 now make the left side a perfect square by halving the linear coefficient, squaring that, then adding it to both sides of the equation...(10/2=5, 5^2=25)
x^2+10x+25=-10 now the left side is a perfect square...
(x+5)^2=-10 now you can move the constant back in...
(x+5)^2+10 you can see that a=1, k=10, and h=5
So h=5