For the first question,
Use the hook's law, F=kx
apply it to the static scenario
F=5N, x=14.813=1.8cm then,
5=1.8k => k=5/1.8 N/cm
now lets consider the dynamic motion and apply newton's second law to the mass which is below the point C
F_sW=ma where F_s is force by the string and W is the weight
klmg=mrw^2
isolate l which is the extended length of the string,
l=m(rw^2+g)/k w is found in the part i
