a) `P+ (3x^2 - 4+ 5x) = x^2 - 4x`
`P= x^2 - 4x - (3x^2 -4 +5x)`
`P= x^2 - 4x - 3x^2 + 4 - 5x`
`P= (x^2 - 3x^2) - (4x+5x) +4`
`P= -2x^2 - 9x +4`
b) `Q- 14y^4 + 6x^5 - 3 = -12y^5 + y^4 -1`
`Q= -12y^5 + y^4 -1 + 3 - 6x^5 + 14y^4`
`Q= -12y^5 + (y^4 + 14y^4) - 6x^5 - (1-3)`
`Q= -12y^5 + 15y^4- 6x^5 +2`