4/ `S=2+2^2+2^3+2^4+...+2^15`
`=(2+2^2+2^3)+(2^4+2^5+2^6)+...+(2^13+2^14+2^15)`
`=2.(1+2+2^2)+2^4(1+2+2^2)+...+2^13.(1+2+2^2)`
`=7.2+7.2^4+...+7.2^13`
`=2.7.(1+2^3+...+2^12) vdots 7`
5/ `6x-4 vdots 2x+1`
`<=>3(2x+1)-7 vdots 2x+1`
`<=>7 vdots 2x+1`
`<=>2x+1 in Ư(7)={1;7;-1;-7}`
`<=>2x in {0;6;-2;-8}`
`<=>x in {0;3;-1;-4}`