`a)3x²y³z^4 .(-4xy³z²)`
`=[3.(-4)].(x².x).(y³.y³).(z^4 .z²)`
`=-12x³y^6z^6`
`b)-12x³y²z.(-2xyz²).5xy²`
`=[(-12).(-2).5].(x³.x.x).(y².y.y²).(z.z²)`
`=120x^5y^5z³`
`c)8/3x³y².(-6x²yz)=[8/3.(-6)].(x³.x²).(y².y).z=-16x^5y³z`
`d)(2x²y)².(-5x³y)`
`=4x^4y² .(-5x³y)`
`=[4.(-5)].(x^4.x³).(y².y)`
`=-20x^7y³`