F = -k(yi+xj)
W = F.dr (dot product of force & displacement)
displacement vector(dr) is of line joining initial & final position vectors...
W = -k(yi+xj).(dxi+dyj)
dW = -k(ydx + xdy)
dW = -kd(xy) (exact)
integrating both sides ,
W = -k [xy] lim from (0,0) to (a,a)
W = -ka2
Answer