How to simplify the expression (x+4)*(3x+5)

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. :)