WHy would it be
important to avoid restriction enzymes that cut the host DNA at a replication site?

the correct answer is - ability to find a mate

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)