Chapter 4 - Polynomials - 4.7 Polynomials in Several Variables - 4.7 Exercise Set - Page 284: 58

$0.04-r^2t^2$

Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $ (rt+0.2)(-rt+0.2) ,$ simplifies to \begin{array}{l}\require{cancel} (0.2+rt)(0.2-rt) \\\\= (0.2)^2-(rt)^2 \\\\= 0.04-r^2t^2 .\end{array}