Find fyxy answer?

Hints: 

If you mean the partial derivative of  f  w/r  to y,  [  ∂f / ∂y ]  , then the partial deriv of   f_y  w/r  to  x,  then the partial derivative of  f_yx   w/r  to y  then do it in stages..          w/r =  with respect to

[ Don't forget to choose a Best Answer... hope no one just gives you all the steps and solution.. you won't learn then ]

f_y =  differentiate the y values only  treat everything else, incl. x  , as a constant     f_y =  5 x^8 y^4  - 4 x^7

Now  take this answer  and  take deriv  of  x,  treat y and  5, and 4 as constant  [ you do this step ]

Now  take that answer, which is f_yx   and  take deriv of  y again, treat x and other terms as constants....

I got    f_yxy =  #  x^7 y^3     you find the #

{  BTW    f_xyy  and  f_yyx  also same exact answer  as  above... so order does not matter ... There may be conditions where the order matters, I don't recall what those condition are :(  }