When a 4.00-kg object is hung vertically on a certain light spring that obeys Hookes law, the

Question:

When a 4.00-kg object is hung vertically on a certain light spring that obeys Hooke’s law, the spring stretches 2.50 cm. If the 4.00-kg object is removed,
(a) How far will the spring stretch if a 1.50-kg block is hung on it, and
(b) How much work must an external agent do to stretch the same spring 4.00 cm from its unstretched position?