If the number is represented by n, your expression is
(n-5)/13
The parentheses are required, unless you write it as
n/13 - 5/13
The wording "the quotient of A and B" is somewhat ambiguous. It does not specify which is the numerator and which is the denominator (or divisor and dividend, as the case may be). Thus, another possible answer is
13/(n-5)
(n-5)/13
The parentheses are required, unless you write it as
n/13 - 5/13
The wording "the quotient of A and B" is somewhat ambiguous. It does not specify which is the numerator and which is the denominator (or divisor and dividend, as the case may be). Thus, another possible answer is
13/(n-5)