Combinding like terms

Question

12xxy^2zz+2x^2+12x^2y^2+9xx+z^2x^2y^2-2xx

Answer 1

12 x^2 y^2 z^2 + 2x^2 + 12 x^2 y^2 + 9x^2 + x^2 y^2 z^2 - 2x^2 =

12 x^2 y^2 z^2 + 12 x^2 y^2 + 9x^2 + x^2 y^2 z^2 <--- 2x^2 cancels

13 x^2 y^2 z^2 + 12 x^2 y^2 + 9x^2 <--- combines x^2 y^2 z^2