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 ?