Chapter 1 - Introduction to Algebraic Expressions - 1.6 Subtraction of Real Numbers - 1.6 Exercise Set - Page 50: 130

$3.8$

The given description, "the difference between $-2.1$ and $-5.9$" translates to \begin{array}{l}\require{cancel} -2.1-(-5.9) .\end{array} Since the expression $a-(-b)$ simplifies to $a+b,$ the expression above simplifies to \begin{array}{l}\require{cancel} -2.1-(-5.9) \\\\= -2.1+5.9 \\\\= 3.8 .\end{array}