Question

product of (5x+4)(5x-4)

Answer 1

Problems like these are easier.
(5x+4)(5x-4) = FOIL
25x^2 (F, First)
-20x (O, Outside)
-20x (I, Inside)
-16 (Last)

(5x+4)(5x-4)
^ ^ First
First, 5x times 5x is 25x^2, what's in ( ) is the previous step!
(5x+4)(5x-4)
^ ^ Outside
(25x^2)-20x, 5x times -4 is -20x
(5x+4)(5x-4)
^ ^ Inside
(25x^2-20x)+20x +4 times 5x is 20x
(5x+5)(5x-4)
^ ^ Last
(25x^2-20x+20x)-16, remove ( ), your answer is 25x^2-16 because -20x and +20x cancel out.

A quicker way to solve is, (5x+4)(5x-4), multiply the First and last, 25x^2-16, ONLY if the problems are like this.