Answer
$25r^{2}-16s^{2}$
Work Step by Step
From Page 301, we know that the product of a sum and difference of two terms $x$ and $y$ is given by the following equation: $(x+y)(x-y)=x^{2}-y^{2}$.
Therefore, $(5r+4s)(5r-4s)=(5r)^{2}-(4r)^{2}=5^{2}r^{2}-4^{2}s^{2}=25r^{2}-16s^{2}$.
