Answer:
9x^2 -49
Step-by-step explanation:
We recognize that this is of the form
(a+b)(a-b) = a^2 - b^2
3x = a and 7 =b
(3x)^2 - 7^2
9x^2 -49
Use a ruler, line the end of the book up with the zero on the ruler, look at the length, boom, have ur answer