The math problem: if she writes 6 checks for exactly $19.98, what expression describes the change in her bank account?

my thoughts: i know the answer is 6(-19.98)

my question: what key word in the problem shows that each check is $19.98, and not the 6 checks in all?

Your key "word" is the parentheses ( ) that encloses the -19.98.  In this case, think of the parentheses as saying multiplication.  It means that in all equations, btw.
so, I write six checks at 19.98.   The negative sign in front of the 19.98 signifies a decrease in the checking account for each check written