How do you write the expression for fifteen equals three more than six times a number and 6?

Hint: First of all this is a very simple problem and a very easy problem. Given a mathematical expression which is an equation which tells that the number fifteen equals to three more than six times a number and the number 6. Here to write the given expression mathematically, first write the left hand side of the equation and then the right hand side of the equation, and then equate it at last.

Complete step-by-step solution:
Given the information that there is an expression. This expression tells that the number fifteen is equal to three times more than six times a number and 6.
Here from the given information, as given fifteen equals to something, hence here the left hand side is the number 15.
Here the left hand side of the equation is 15, which is expressed mathematically below:
$ \Rightarrow 15 = ......$
Now we have to mathematically express the right hand side of the given equation.
Given that the right hand side is equal to three more than six times a number and a 6.
Here let the number be $x$.
Now given that the right hand side is equal to the number 3 more than six times the assumed number $x$ and 6.
The right hand side of the equation is expressed mathematically below:
$ \Rightarrow 3 + 6\left( {x + 6} \right)$
This is the right hand side of the equation as given that it is 3 more than six times the number $x$ plus the number 6.
Hence the simplified expression of the right hand side is:
$ \Rightarrow 3 + 6\left( {x + 6} \right)$
$ \Rightarrow 3 + 6x + 36$
Now equating the left hand side of the equation and the right hand side of the equation.
$ \Rightarrow 15 = 3 + 6\left( {x + 6} \right)$
$ \Rightarrow 15 = 3 + 6x + 36$

The expression for fifteen equals three more than six times a number and 6 is $15 = 3 + 6x + 36$.

Note: Please note that while expressing the right hand side of the equation. As it is given that 3 more than six times a number and 6, then it could be $15 = 3 + 6x + 6$, but no, this is not true. We have to write the number 6 along with number $x$, which is taken as $\left( {x + 6} \right)$, hence the equation becomes to $15 = 3 + 6\left( {x + 6} \right)$, which is the equation $15 = 3 + 6x + 36$.