Converting standard form to vertex form

when 7x^2-28x-5 is written in the form a(x-h)^2+k, k=?

so its like converting a standard form equation to vertex form(right?)
so i try to do axis of symmetry and get 2, what would this represent in the 2nd equation?
any help on solving this? thanks

ax^2 + bx + c = a(x^2+bx/a) + c
= a(x^2 + 2*.*(b/(2a)) + (b/(2a))^2 - (b/(2a))^2) + c
= a(x + b/(2a))^2 + c - b^2 / (4a)
= a(x + b/(2a))^2 - (b^2-4ac)/(4a)

If you can get it, just remember :
a.x^2 + b.x + c = a(x-h)^2 + k
Where : h = -b/(2a) , k = -(b^2-4ac)/(4a)

7x^2 - 28x - 5
= 7(x^2 - 4x) - 5
= 7(x^2 - 2.x.2 + 2^2 - 2^2) - 5
= 7(x-2)^2 - 7.2^2 - 5
= 7 (x-2)^2-33