Denote the right hand sides of the equation by
f1(x1,x2) and
f2(x1,x2) respectively.
(x1,x2) in R2 is an eqm point if
f1(x1,x2)=0 and
f2(x1,x2)=0.
(1) f1(x1,x2)=0 => x1=0 or
ax2+bx1=br.
(2) f2(x1,x2)=0 => x2=0 or
ax1-bx2=df.
Since we want both (1) and (2) to be satisfied, this can happen in any
one of 4 ways (each condition from (1) can be paired with any one of 2 conditions from (2)).
Therefore the four possible solutions are
(a) x1=0 and x2=0 (or)
(b) x1=0 and ax1-bx2=df (or)
(c) ax2+bx1=br and x2=0 (or)
(d) ax2+bx1=br and ax1-bx2=df
You found solutions (a) and (d).