The given binomial is
9t^2 - 4
Recall,
4 = 2^2
9 = 3^2
Thus, it can be rewritten as
3^2t^2 - 2^2
(3t)^2 - 2^2
We would apply the difference of two squares whcih is expressed as
(A^2 - B^2) = (A + B)(A - B)
By comparing both expressions,
A = 3t and B = 2
Thus, the factored form of the binomial is
(3t + 2)(3t - 2)