Write a linear equation in two variables to represent the following statement.

The cost of a book is thrice the cost of a pen.

First, assign two variables for the cost of a book and the cost of a pen. Then form an equation according to the given statement.

Let the cost of a book be x and the cost of a pen be y. 

According to the given statement, the cost of a book is thrice the cost of a pen
$\Rightarrow x=3y$

$\Rightarrow x-3y=0$

 which is a linear equation in two variables

Thus, the linear equation in two variables to represent the given statement is

$\Rightarrow x-3y=0$