There are two ways that can be used to name isotopes of an element. This is best illustrated by examples. We'll use the element chlorine for simplicity.
The first example is the more common one. The formula for this notation is E-m where E is the name of the element and m is the mass number.
(1) Chlorine-35
(2) Chlorine-37
(3) Chlorine 38
Another is less commonly used. The formula for this notation is m^ a L where m is the mass number, a is the atomic number and L is the first letter of the element.
(1) 35^ 35 Cl
(2) 37^ 35 Cl
(3) 38^ 35 Cl