21!/9!12! how do i do this ?

You can use the calculator here to do this:

$${\frac{{\mathtt{21}}{!}}{\left({\mathtt{9}}{!}{\mathtt{\,\times\,}}{\mathtt{12}}{!}\right)}} = {\mathtt{293\,930}}$$

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An exclamation point means the product of the integers from 1 to n. For example 4! means 4*3*2*1