http://prntscr.com/lfphdu
You don't need to multiply all this out
Just look at the denominator. It's not allowed to be zero.
\(d = (5k^2 - 25k) \cdot 7k^4 = \\ 35k^5(k-5)\\ \text{so }k\neq 0,~5\)
so (a) and (d)
