Explain the difference betweena theory and a
law.

Theories are ideas and supposition used to explain something, a law is a system of rules in place to regulate actions of their members.  Theories are something that can be bent and changed at will. Laws have to be judged in court before someone can change them.

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.

: -)

yes it is weird but true