Start with x, divide by 6,then subtract 1

Here in the above given expression, X can be any number hence it is a variable.6 and 1 are constant because they cannot be changed.

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction