Ineed on a algebra word

there is a rat and a mouse.
the rat is x years old.
the mouse is 15 years younger than the
how to convert that into a math

Step #1: 
Make up symbols for the age of the rat
and the age of the mouse.

They already told you that 'x' is the age of the rat.
Let's say that 'M' is the age of the mouse. 

Step #2:
Translate the words into a math sentence,
something like this:

"The mouse is              15 years younger than      the rat."
             \/                                    \/                                \/  
"The mouse's age is    15 years less than            the rat's age."

The mouse's age      is        the rat's age          but 15 years less.
           \/                    \/                \/                          \/
           M                   =                x                        - 15

                            M  =  x - 15

Does this help you see it ?