What is 9a squared plus 42a-49 factored?

Are you sure it's minus 49?

If so, then:

9a2 +42a -49

={(3a)2 + 2·3a·7 + 72} -2·72

=(3a -7)2 - (7·sqrt(2))2 (2nd binomial formula)

={(3a -7) + (7·sqrt(2))} · {(3a -7) - (7·sqrt(2))} (3rd binomial formula)

=(3a +2,89949) · (3a -16,89949).

Otherwise if it's plus 49 then simpler:

9a2 +42a +49

={(3a)2 + 2·3a·7 + 72}

=(3a -7)2.