Nearly every function (unless it is very small) should begin with a let:
(define (myfun args )
(let ( (var_1 value_1 ) ... )
code ))
Rule: Every local variable used in a function must be declared, i.e., must be either an argument of the function or a let variable.
Declaring let variables is similar to a var declaration in Pascal.
If you fail to declare a let variable, the variable is treated as a global variable. This is a dangerous practice that will be penalized in grading.