Use a special product formula to factor the perfect square trinomial?

Question

9x^ - 42x + 49

Answer 1

(3x - 7) (3x - 7)

You can check that by multiplying it out.

Answer 2

9 = 1*9, 3*3, or 9*1
49 = 1*49, 7*7, or 49*1
For the moment pretend 42 has a + sign (makes typing easier)
1*49 + 9*1 = 58, too big
9*49 + 1*1, too big
9*7 + 1*7, too big
3*7 + 3*7 = 42, aha!
Now, remember the - sign
3(-7) + 3(-7) = -42
OR
(-3)7 + (-3)7 = -42
so
(3x - 7)(3x - 7)
OR
(-3x + 7)(-3x + 7)