Answer:
Step-by-step explanation:
It appears to be the last step there that has evaded you. In the second line of work, you can see that what's in each set of parenthesis is exactly the same. Because of this, it is common and can be factored out. What's left is the 4x and the +5:
(5x - 3)(4x + 5)
That's factoring by grouping. It only works with polynomials that have an even number of terms so you can group them together in 2's.
