If you needed to add $$-18$$ and $$-7\text{,}$$ note that both are negative. Maybe you have this expression in front of you:

\begin{equation*} -18+-7 \end{equation*}

but that “plus minus” is awkward, and in this book you are more likely to have this expression:

\begin{equation*} -18+(-7) \end{equation*}

with extra parentheses. (How many subtraction signs do you see? How many negative signs?)

Since both our terms are negative, we can add $$18$$ and $$7$$ to get $$25$$ and immediately realize that our final result should be negative. So our result is $$-25\text{:}$$

\begin{equation*} -18+(-7)=-25 \end{equation*}
in-context