A + B = E ......... (1)

B - A = C .......... (2)

A - D = Q ........... (3)

D - B = L ............. (4)

taking eqn (1) - eqn (2) gives ;

2A + C = E ................. (5)

then eqn (3 ) - eqn (4) gives ;

A + B = Q - L ............... (6)

last taking eqn (5) - eqn (6) gives ;

A + C - B = E + L - Q ; which is the required equation

So;

A + C - B = E + L - Q