Question

Determine all pairs (f, g) of functions from the set of real numbers to itself that satisfy
g(f(x + y)) = f(x) + (2x + y)g(y)
for all real numbers x and y.
A4

Answer 1

The A4 was the problem number.

Answer 2

*

Answer 3

Solution is

f(x) = x2 + C and g(x) = x

where C is a real number.

but its hard to understand.