Work done in second stretching = Potential energy after second streching - Potential energy after first stretching
Potential energy stored, when it is extended by x, is
U1= (1/2) k *x²
Potential energy stored, when it is further extended by y, is
U2 = (1/2) k* (x+y)²
Work done = U2 - U1
= (1/2) k [x² +y² + 2xy - x²]
= (1/2) k*y ( 2x +y)
Answer : (1/2)ky(2x+y)