What about it is not understandable?

Basically I don't know to get to this step. the answer is from the teacher.

Its basically factoring out common factors.
They have 2(x + 2y)(2y - x) in common.

10(x + 2y)(2y - x)² - 12(x + 2y)(x - 2y)

= 5(2(x + 2y)(2y - x)(2y - x)) - 6(2(x + 2y)(x - 2y))

= 5(2y - x)(2(x + 2y)(2y - x)) + 6(2(x + 2y)(-(x - 2y))

= 5(2y - x)(2(x + 2y)(2y - x)) + 6(2(x + 2y)(-x + 2y))

= 5(2y - x)(2(x + 2y)(2y - x)) + 6(2(x + 2y)(2y - x))


Now you factor out 2(x + 2y)(2y - x).

= 2(x + 2y)(2y - x)(5(2y - x) + 6)

= 2(x + 2y)(2y - x)(10y - 5x + 6)

What you also need to recognise is that :

That - 6 can be changed into + 6 by introducing a negative term in the inside


And,

-(x - 2y) = -x + 2y = 2y - x


Your teacher didn't show the intermediate steps, but that's probably because you're expected to already see what needs to be factored out and thus she only gave you the subsequent two steps.