write the quadratic function f(x)-x^2+14x-50 in the form f(x)=a(x-h)^2+k by completing the square and graph

I don't know if that is

f(x) = - x^2 ...
or
f(x) = x^2 ...

Since you needed an equation, I will assume that the - sign was supposed to be an = sign
If not, make the necessary changes, by factoring out the -1

y = x^2 + 14x - 50
= x^2 + 14x +49 - 49 - 50
= (x+7)^2 - 1

I will leave the graphing to you.