the sentence given is
add the product of 3 and 6 to 4
we have been asked to add a particular number to 4
so we add 4 to the particular number
the number we are adding 4 to is the product of 3 and 6
when we say a product of numbers, product is the number you get after multiplying the 2 numbers
product of 3 and 6 is written as 3 x 6
and we are adding 4 to the product so we write the expression as
(3x6) + 4