Question 1074223
f(x) = 12 - (x^2 / 2);
If x=2k, then f(2k) = 2k;
f(2k) = 12 - ((2k)^2 / 2) = 12 - (4k^2 / 2) = 12 - 2k^2;
12 - 2k^2 = 2k;
12 = 2k^2 + 2k;
6 = k^2 + k; or k^2 + k = 6;

It is true only if k = 2, then 2^2 + 2 = 4 + 2 = 6;