kx = x !/r! (x – r)! kx− r
is that right?
The right side of your equation is confusing. Put in some parentheses.
see attachment -
The thunbnail is hard to read. In the second equation there are terms with k to some exponent. It is hard to read these exponents. Also, what is the question - prove the second equation holds?
Separate names with a comma.