There actually are a handful of functions f(x) and g(x) that make this work!
Anything where you can combine the function f(x) and g(x) to get f(g(x)) = 5x + 11
for example if you picked
f(x) = x and g(x) = 5x + 11 that would work
because f(g(x)) would be f(5x + 11) = 5x + 11
but similarly so would
f(x) = x + 11 and g(x) = 5x
because f(g(x)) = f(5x) = 5x + 11
so you just need to pick some combination of functions where that property would hold
I hope that helps! :)