6)
The expression 2(r+3) can be written using distributive property as,

(Distributive property, A(B+C)=AB+AC).
Therefore, 2(r+3)=2r+6.
7)
The expression 6(4s-1) can be written using distributive property as,

Therefore, 6(4s-1)=24s-6.
8)
8t+2 can be written as,

Therefore, 8t+2=2(4t+1).