What is f of x is equal to 7 x squared plus 42 x written in vertex form?
A
f of x is equal to 7 times open paren x plus 6 close paren squared minus 42B
f of x is equal to 7 times open paren x plus 6 close paren squared minus 6C
f of x is equal to 7 times open paren x plus 3 close paren squared minus 9D
f of x is equal to 7 times open paren x plus 3 close paren squared minus 63