so the 2 equations are
x-m=l and
(l-m)x+a=0
assuming that (l-m)x means (l-m) times x
x-m=l and
(l-m)x=a
solve for x in first equation
x-m=l
add m to both sides
x=l+m
subsitute (l+m) for x in other equation
(l-m)x+a=0
(l-m)(l+m)+a=0
l²-m²+1=0
there, x is eliminated
