strain ennergy release rate+stress intensity factors

I'm having a hard time trying to relate the the forces specified in the figure so that I can obtain the expression for the strain energy release rate.

For the 'L-opening load' P when acting alone, the energy release rate is given by: 

 G= (12*P²*L²) / (E*B²*h³)

For the '2L/3-opening load' P when acting alone, the energy release rate is given by:

 G=(16*P²*L²) / (3*E*B²*h³)