As per exponent rule, at dividing of powers with the same base their exponents are subtracted i.e.
am
---- = am-n
an
let us suppose am = a5 and an = a3
Now a5 = a.a.a.a.a and a3 = a.a.a
On dividing, we get a5 / a3 = a.a.a.a.a / a.a.a = a.a = a2 or a5-3
am
Thus, we can say, ---- = am-n
an