What is f(x) = 7x2 + 42x written in vertex form?

Complete the square
factor out linear coefient
f(x)=7(x^2+6x)
take 1/2 of linear coefient and square it and add negative and positive to inside
6/2=3, 3^2=9

f(x)=7(x^2+6x+9-9)
complete squre
f(x)=7((x+3)^2-9)
expand
f(x)=7(x+3)^2-63 is veretx form
vertex is (-3,-63)