Eliminate "x" from equation x - m = l (l - m) x + a = 0

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