When a $2\mathrm{.}5-$kg object is hung vertically on a certain light spring described by Hooke$$s

law, the spring stretches $2\mathrm{.}76$cm$.$

a$.$ What is the force constant of the spring?

b$.$ If the $2\mathrm{.}5-$kg object is removed, how far will the spring stretch if a $1\mathrm{.}25-$kg block is hung on it$?$

c$.$ How much work must an external agent do to stretch the same spring $8\mathrm{.}0$ cm from its unstretched position?