4000 + 2000 = 0 ... ? ? ?

[Blade™]

let a = 4000,

b = 2000 and

c = 6000

a + b = c

Multiply both sides by a + b

(a + b) (a + b ) = c (a + b)

a*a + ab + ba + b*b = ca + cb

a*a + ab - ca = cb - b*b - ba

a( a + b -c) = -b(b + a - c)

so a = - b

a + b = 0

Hence 4000 + 2000 = 0.

Ok, where is the mistake. I mean, where is the wrong step. This can't be.