Derivative of Polynomials

Suppose f is a function which satisfies f(0)=π and f(x+y)=f(x)+f(y)f'(0)=\pi \text{ and }f(x+y)=f(x)+f(y) for all x and y. True or false: f(x)=πxf(x)=\pi x satisfies this function.
More The Power Rule (Derivative of Polynomials) Questions: