alright, so continuing from the previous post where we know the value of C
solving for K2 yields the equation
(2/3)x^2/(c-x)^3=K2, it's completely in terms of x
so [Al3+]=(2/3)x^2 for equation 2 and (4/9)x^2 for number 1.
the answer is equation number 2. This situation is for where K2=K1, this assumption was made in the previous post. Now if K2>K1, it would increasingly favor equation 2. Try solving it out for yourself, the (c-y)^3 cancels out fortunately, to yield an equation completely in terms of x.
There may have been a more simpler way to solve it, but this is quite simple enough for me. Also, are you sure that your OP had the exact form of the question? The question seems quite useless.