Answer
(3x+7)(3x-7)
Work Step by Step
The student made the error that they did not simplify the $9x^{2}$ as in a difference of squares formula both first and last terms are simplified.
The difference of squares formula is:
$(a-b) (a+b) = a^{2} - b^{2}$
= $9x^{2}$ - 49
*** Take the square root of $9x^{2}$ which is 3x. Becuase 3x × 3x= $9x^{2}$
*** Take the square root of 49 which is 7. Becuase 7 × 7= 49
= $(3x)^{2}$ - $(7)^{2}$
In the given formula let 3x represents a and 49 represents b.
$(3x)^{2}$ - $(7)^{2}$ = (3x+7)(3x-7)
