sorry. not familiar with the box method. here's another way to do it.

multiply the first term by the lone constant, we get 3x^2*(-18) = -54 x^2

now take the middle term. we have -25x

so now we want two terms that when we multiply, we get -54x^2 and when we add we get -25x

thinking a bit ....aha! 2 and -27. since 2 + (-27) = -25 and 2*(-27) = -54

so now we split the middle term into those two numbers. so we go from

3x^2 - 25x - 18

to

3x^2 - 27x + 2x - 18

now we factor by groups. there is a common 3x among the first two terms, and a common 2 among the last two, so pull those out. we get:

3x(x - 9) + 2(x - 9)

now we have a common (x - 9) to both terms, so pull those out. we get

(x - 9)(3x + 2)

So, 3x^2 - 25x - 18 = (x - 9)(3x + 2)

This is one of the standard ways to do this kind of problem, you always follow these steps exactly. if you prefer the boxed method. post a full solution of a problem you worked out using it, and i'll figure it out and explain it to you