Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.3 - Integers: Addition and Subtraction - Problem Set 1.3 - Page 20: 73

$18$

The top number is $\text{ negative }$ and the bottom number is $\text{ positive .}$ Since the signs of the numbers are different, subtract the numbers and copy the sign of the bigger number. Hence, \begin{array}{r}\require{cancel} -21 \\\underline{\phantom{-} 39 }\\ 18 \end{array}