Simplify combining like terms:

(i) 21b - 32 + 7b - 20b

(ii) - z^{2 }+ 13 z^{2} - 5 x + 7 z^{3} -15z

(iii) p - ( p - q ) - q - (q - p)

(iv) 3a - 2b - ab - (a - b + ab ) + 3ab + b - a

(v) 5 x^{2}y - 5 x^{2} + 3 yx^{2} - 3y^{2} + x^{2} - y^{2} + 8 xy^{2} - 3y^{2 }

(vi) (3 y^{2} + 5 y - 4 ) - (8 y - y^{2} - 4)