Edward C. answered 10/12/19
Caltech Grad for math tutoring: Algebra through Calculus
Hi Oscar,
This is just factoring out the GCF (Greatest Common Factor) from an expression with 2 terms. It might be easier to see what's happening if you let A=x+6 and B=x+3. Then your original expression is equal to
5A7B4 + 7A6B5
The GCF is A6B4, so factor it out to get
A6B4(5A + 7B)
Now 5A + 7B = 5(x+6) + 7(x+3) = 5x + 30 + 7x + 21 = 12x + 51 = 3(4x+17), so the expression is equal to
3(x+6)6(x+3)4(4x+17)