No on can explain it but it has a definition.
Kx+9=7
Kx=-2 (subtract 9 from both sides)
X=-2/k (divide k from both sides.
You know that
.
Use the property
to state that

Then the expression
is equivalent to

Answer: correct choice is A.
1, X = 6
2, X = 13
3, X = 9