Question

Factor completely 49x^2-36

Answer 1

(7x - 6)(7x + 6)

There is a positive and a negative square root to every number. 
Hope this is helpful. :)

Answer 2

(7x+6)(7x-6)
All three of the factors in the binomial are square roots. The square root of 49 is 7, the square root of x^2 is x, and the square root of 36 is 6. Also, since you are subtracting, you know that you will have a positive and a negative factor in the end.