when you stretch a spring 13 cm past it's natural length, it excerts a force of 21 N. What is the spring constant of this spring?

Formula

F = k*x

Givens

F = 21 N

x = 13

K = ?

Solution

21 = 13 k

21/13 = k

k = 1.615 N / cm

The units are not quiet the ones you should be using. Newtons are found in the mks system.

13 cm [1 m / 100 cm ] = 0.13 m

So an alternate solution is

21 N = k * 0.13 m

k = 21 N / 0.13 m

k = 161.5 Newtons / meter. Answer