Depending on the values of r, t, and e, the numerical value of that expression
might have many factors.
For example, if it happens that r=5, t=1, and e=4 for an instant, then, just
for a moment, (r + t)(e) = (5+1)(4) = 24, and the factors of (r+t)(e) are
1, 2, 3, 4, 6, 8, 12, and 24. But thats only a temporary situation.
The only factors of (r+t)(e) that dont depend on the values of r, t, or e,
and are always good, are (r + t) and (e).