We're calling our "number" as b. We want to transform the sentence "Nine less than the product of 3 and a number is equal to 8" into an equation. From the first part "Nine less than the product of 3 and a number", we have our number multiplied by 3, and then subtracted 9 from it.
From the end of the sentence "is equal to 8", this means that this expression written is equal to 8.
And this is the equation.