You use PEMDAS.
First, parentheses
(x+4) = 4x
(3x+5) = 3x+5
So, that is
4x*3x+5
Then, exponents. We don’t have any.
Then, multiplying.
4x*3x is 12x
So, 12x+5
And then, division. We don’t have anything to divide.
Then, addition and subtraction
You can’t add the variable to a number, so the answer is just
12x+5.
Hope it helped. :)