You have to write the declaration and determine the value for thrust in the code that you write. For example, in Java, you can just say
int thrust = 100;
If the print statement is looking for a variable called "thrust" it should find it and print 100.
I believe the default print statement in Java (for my league, yours might be different) is:
System.out.println("8000 4500 100");
The "100" is the default thrust. To print out your variable, you change it to:
System.out.println("8000 4500 " + thrust);
If you have also defined x and y you might use:
System.out.println(x + " " + y + " " + thrust);
When coding game runs your code, it will see your output of
8000 4500 100, and use that to calculate your next position. The game is setup as a loop, where at each step you get input data for your current position and you have to print out a line of
x y thrust
You define what to print in that System.out statement at the end of the loop.