Question 540641
{{{(5x+4)(5x-4)}}} is a sum of two terms/numbers times their difference, and that equals the difference of the squares. They teach you that as a special product, usually with the formula {{{(a+b)(a-b)=a^2-b^2}}} 
{{{(5x+4)(5x-4)=(5x)^2-4^2=5^2*x^2-16=25x^2-16}}}
If you were not taught about special products, you may have to multiply and simplify, maybe like this
{{{(5x+4)(5x-4)=(5x)(5x-4)=4(5x-4)=(5x)(5x)+(5x)(-4)+4(5x)+4(-4)=25x^2-20x+20x-16=25x^2-16}}}
You can amaze people with feats of mental math using difference of squares. How much is 801 times 799? Well, that's
{{{801*799=(800+1)(800-1)=800^2-1^2=640000-1=639999}}}
How much is 25 times 35? {{{25*35=(30-5)(30+5)=30^2-5^2=900-25=875}}}
{{{2x(x+5)+x(3-x)=2x*x+25*5+x*3+x*(-x)=2x^2+10x+3x-x^2=(2x^2-x^2)+(10x+3x)=x^2+13x}}}
{{{5ab^2b(7ab^2+3a-4b)=5a(b^2*b)(7ab^2+3a-4b)=5ab^3(7ab^2+3a-4b)=5ab^3(7ab^2)+5ab^3*3a-5ab^3*4b=35a^2b^5+15a^2b^3-20ab^4}}} 
{{{(5m^2-2mp-6p^2)-2(-3m^2+5mp-p^2)=5m^2-2mp-6p^2-2(-3m^2)-2(5mp)-2(-p^2)=5m^2-2mp-6p^2+6m^2-10mp+2p^2=(5m^2+6m^2)+(-2mp-10mp)+(-6p^2+2p^2)=11m^2+(-12mp)+(-4p^2)=11m^2-12mp-4p^2}}}
The first step was multiplying that -2 times the second parenthesis applying the distributive property. I really see that -2 as +(-2), but I do not always write it out the way I see it. I only do it when I want to avoid confusing myself.  Later on, you'll notice that I changed -2mp to +(-2mp). I did that to avoid confusing myself. I believe algebra would be easier if everyone realized that subtraction does not really exist, and 7-3 is really 7+(-3). If you take the sign as part of the number, you get less confused. When you see a minus sign in front of a parenthesis, as in {{{-(x-3y)}}} take it to mean there is a +(-1) instead of the minus sign to state +(-1)(x-3y).
{{{(3x^2y^6)(-4x^2y^6)=((3)(x^2)(y^6))((-4)(x^2)(y^6))=((3)(-4))((x^2)(x^2))((y^6)(y^6))=(-12)(x^4)(y^12)=-12x^4y^12}}}