so you need to simplify the two fractions by first simplifying the exponent. this would mean that each deviant has one set of exponent instead of a pair.
thus, -4`40 ÷ -4`21
the two bases are equal.
#4 rule states that when a`n/b`m (fraction). if bases are the same, the exponent of the denominator is subtracted to the exponent of the numerator. assuming a & b are equal, the answer will look like this: (a`(n-m))
so apply this rule to your question. also, it'll be helpful to learn the laws of indices.
final answer: 4`19
NOTE: ` stands for 'the power of`
so in word form your answer is four to the power of nineteen.
Hope this helped :)