To solve the example you should know important things:
firstly: what is variable? a variable is a place in memory where you can store data in.
Secondly: how to assign value to variable? by put the variable name in the left of equal operator and the value you want to store it in the right of equal operator.
Here is the rule:
Variable name = assign value you want to store temporary
var data = 5; //this mean that you put 5 in the data variable
If you stuck, here is the solution of answer try to understand what i wrote it from the solution:
var b = 2;
// Only change code below this line
a = 7;
b = a;
Hope this help you to understand and solve the example, good luck and have a happy coding.