Answer:
F = - K x for spring (note that that F here is given in grams, F = m g is correct)
K here is 100 g / cm for the spring constant
x = -420 g / 100 g/cm = -4.2 cm
The spring would compress 4.2 cm for a total length of 20 - 4.2 = 15.8
d) to compress the spring 6.8 cm one can see that the load would be 680 g
