Let us assume x be an unknown number.
Given that "three less than the product of a number and six".
Product represents operation of multiplication and less than represents operation of subtraction.
Product of unknown number x and 6 = x * 6 = 6x.
Three less than 6x could be written as subtracting 3 from 6x that is 6x-3.
Therefore, three less than the product of a number and six = 6x-3.