Consider the functions p(x) = 2x + 5 and k(x) = 3x + b, where b is a real number. If p(k(x)) = k(p(x)), what does b equal?
p(k) = 2k+5
k(p) = 3p+b
2k(x)+5 = 3p(x)+b
b = 2k(x)-3p(x)+5
Unless you have some further information on the functions p and k, that's as far as we can go.