20/x2-4

Please use parentheses to clarify your question! It seems you want to differentiate:

(\frac {20} x ) 2 - 4

But perhaps you mean:

\frac {20} {x^2} - 4

Or maybe even:

\frac {20} {x^2-4}

In any event please show us your attempt and we'll help if you get stuck.