xz + 2x = (z+2)x

The distributive principle (used in reverse).

Usually the distributive principle is written

x(z + 2) = xz + 2z

where we say

The x multiplier in front of the parentheses
distributes as a multiplier to the z giving xz,
then it also distributes as a multiplier to the
2 giving 2z.  But x(z+2) can also be written
as (z+2)x by the commutative principle, and
the right and left sides of the equation can
be reversed and we get:

xz + 2x = (z + 2)x

Edwin