What is the factored form of the expression?

k2 – 9h2
a. (k + 3h)(k + 3h)

b. h2(k + 3)(k – 3)

c. (k + 3h)(k – 3h)

d. (k – 3h2)(k + 3)

The equation which is supposed to help you is this one:
a^2 - b^2 = (a+b) (a-b)

So, what we need to do here is apply this equation:
k^2 - 9h^2 = 
k * k - (3h)^2 =
(k + 3h) (k-3h)

The correct answer is C.