Solve for F1:
F1-F2-P=0
F1=F2+P
and substitute the expression for F1 into the compatibility equation:
(F2+P)L1A1E1+F2L2A2E2=0
Solve this equation for F2:
F2 L1A1E1+L2A2E2=-PL1A1E1
F2 =-PL1A1E1L1A1E1+L2A2E2