`a,6x(5x+3)+ 3x(1-10x)=7`
`<=>30x^2+18x+3x-30x^2=7`
`<=>21x=7`
`<=>x=1/3`
`b,(3x-3)(5-21x)+(7x+4)(9x-5)=44`
`<=>15x-63x^2-15+63x+63x^2-35x+36x-20=44`
`<=>79x-35=44`
`<=>79x=79`
`<=>15x-63x^2-15+63x+63x^2-35x+36x-20=44`
`<=>79x-35=44`
`<=>79x=79`
`<=>x=1`
`c,4(18-5x)-12(3x-7)=15(2x-16)-6(x+14)`
`<=>72-20x+36x+84=30x-240-6x-84`
`<=>156-56x=30x-240-6x-84`
`<=>156-56x-24x+234=0`
`<=>480=80x`
`<=>x=6`
`d,(x+2)(x+3)-(x-2)(x+4)=16`
`<=>x^2+3x+2x+6-x^2-4x+2x+8=16`
`<=>3x=2`
`<=>x=2/3`
Vậy .......
