Try using the box method. When I do it, it makes multiplying trinomials and binomials like these easier.
Here is how I would set up the equation:
as you can see in the picture below I multiplied x by 3x^2, -4x, and positive six which gives me a product of 3x^3 (when multiplying always add the exponents) -4x^2 and 6x. Afterwards I did the same thing by multiplying one to the trinomials (3x^2, -4x and 6). Now to simplify the answer you will add by like terms. So -4x^2 plus 3x^2= -x^2
6x+(-4x)= 2x. Since the only two numbers left (6 and 3x^3 aren’t like terms, you leave it like that. Your answer will be 3x^3 + (-x^2)+2x+6.