When you stretch a spring 13 cm past it's natural length it exerts a force of 21 N. what is the spring constant of this spring

The formula for Spring Constant is K = (F/X), with K being Spring Constant, F being force, and X being extension (in meters preferably).

This means the first thing you need to do is convert your 13 cm to meters, which you do by moving the decimal place over to the left two spaces. This obviously gives you 0.13m

Now all you have to do is divide your force by your extension, and 21/0.13 comes out to your answer: 161.53 N/m